**Background:** Ruminants have evolved diverse nutritional adaptation strategies in response to environmental changes, classified as concentrate selectors, mixed feeders, or roughage feeders. Blue sheep (Pseudois nayaur) are medium-sized wild sheep endemic to the Qinghai-Tibet Plateau and adjacent areas, with the Helan Mountains representing their easternmost distribution range and one of the highest population densities globally (3.627–4.635 sheep/km²). Understanding the morphological and structural characteristics of their digestive organs and how these adapt to seasonal changes is important for understanding their population dynamics and survival strategies.
**Methods:** From 2012 to 2013, 64 adult blue sheep samples were collected from the Helan Mountain National Nature Reserve in Northwest China. External morphological parameters (body weight, body length, tail length, ear length) were measured. The compound stomach was separated and each component (rumen, reticulum, omasum, abomasum) was weighed individually, including contents. Volumes were measured using the irrigation method. Intestinal tract measurements included length and weight of small intestine, cecum, large colon, small colon, and rectum. Rumen surface enlargement factor (SEF) was determined by measuring papillae number, height, and width per square centimeter from samples taken from dorsal and ventral rumen, atrium ruminis, and dorsal blind sac. Feed particle distribution was assessed by sieving contents from different digestive tract regions through graded sieves (x > 2.0 mm, 1.0 mm < x ≤ 2.0 mm, 0.425 mm < x ≤ 1.0 mm, 0.25 mm < x ≤ 0.425 mm, and 0.125 mm < x ≤ 0.25 mm). Statistical analyses included Pearson correlation, linear regression, one-way ANOVA, and ANCOVA using SPSS 20.0.
**Key Results:** The stomach tissue weight was 0.78 ± 0.03 kg (n=61), and stomach weight including contents was 3.77 ± 0.28 kg (n=63). Rumen and reticulum tissue accounted for 80.88 ± 0.45% of stomach tissue weight. Rumen and reticulum weights were positively correlated with body weight (r=0.556, F₁,₂₃=10.307, p=0.004). The proportion of rumen and reticulum contents to body weight ranged from 11% to 20%, consistent with roughage feeders. The overall rumen surface enlargement factor was 2.85 ± 1.37, which is below the threshold of 3.5 characteristic of roughage feeders. There were significant differences in mastoid density, length, and width across different rumen regions (p < 0.001). The total intestinal length was 22.79 ± 0.26 m (n=60), 26.1 ± 0.5 times body length, with the small intestine accounting for 73.06 ± 0.29% of total length. No significant differences were found in food channel aperture, intestinal length, or digestive tract weight between seasons (p > 0.05) or sexes (p > 0.05). However, significant seasonal differences were observed in omasum weight (including contents), abomasum tissue weight, and abomasum (including contents). Small intestine (F₁,₆₀=9.671, p=0.003), small colon (F₁,₆₀=12.434, p=0.001), and rectum weights (F₁,₆₀=5.716, p=0.020) were larger in winter than summer. Feed particle analysis showed that large particles (x > 2 mm) in the rumen and reticulum reached 27.7 ± 3.03% of total feed particles. The proportion of feed particles larger than 1 mm in the omasum was approximately 50%, similar to that of concentrate selectors like moose. The reticulum–omasum passage measured 3.65 ± 0.11 mm.
**Clinical Implications:** This study provides baseline anatomical and physiological data on blue sheep digestive morphology, confirming their classification as roughage feeders according to Hofmann's nutritional adaptation theory. The findings demonstrate that blue sheep employ a fast-feeding, fast-excretion strategy to consume large quantities of low-quality, high-cellulose plants, enabling them to adapt to seasonal environmental changes in arid and semi-arid regions. The seasonal variation in feed particle distribution and organ weights suggests that blue sheep adjust digestive processing rather than passage anatomy to cope with changing forage quality. These results serve as a reference for understanding digestive system adaptations in other ruminant species and inform conservation strategies for blue sheep populations, particularly in the context of habitat and forage availability changes.