Abstract: Based on the theory of the stable isotope technique，this paper determined the relative contributions of different food sources to animal diet (PCV,Eqs3),dietary contributions ratio (PAiP,Eqs7) and the trophic level of small rodent (TLc,Eqs9) in alpine meadow ecosystem: PCVAi=cos(ΔαPAi)ΖPAi(3) PAiP=PCVAi∑I=1PCVAi×%(7) TLc=1+(αc-αTL1)/Δαcd(9) Where:ΔαPAi represents the angle between predator vector (P) and food vector (Ai);ΖPAi represents the Euclidean distances between predator and food;αc is the consumer vector angle;αTL1 is the first trophic level vector angle;Δαcd is a const ant as the isotopic enrichment factor.Simultaneously,the model of preying and trophic relationship between two species was estimated in alpine meadow ecosystem:while cos(Δα)/PCVmin≤ΖS1S2≤cos(Δα)/PCVmax,preying relationship presents in two species and one species eat the other;while ΖS1S2cos(Δα)/PCVmax,preying relationship absents,the former Eqs.shows two species in the same trophic level and the latter in more than one trophic level.In addition,the result,which the trophic level of animal is estimated by using Eqs9 in alpine meadow ecosystem,consistents with Zhang et al (1999).