It is important to grasp the decomposition process of wheat straw and hairy vetch mixed under different bag filling amounts for straw return and soil fertilization in the agricultural area of eastern Qinghai. In this study，two factors，material type and bagging volume were set up by nylon bagging experiment with a total of nine treatments in completely randomized combinations. The decomposition characteristics of the three material types at different bagging levels were investigated at three levels. The three material types were straw（W），hairy camelina（H）and their mixed material（WH），and three bagging level were 16，28 and 44 g. The results showed that the decomposition of organic materials under each treatment showed a change pattern of fast in the early stage-slow in the middle stage-and faster in the late stage，and the material type had a significant effect on the cumulative decomposition rate until the end of decomposition（P＜0.05），and the cumulative decomposition rates of wheat straw，hairy vetch and mixed materials were 45.27%，76.75% and 62.25% on average，respectively，showing that hairy vetch ＞ mixed materials ＞ straw. The bagging volume had a significant effect on the cumulative decay rate（P＜0.05），with the average cumulative decay rates of 62.84%，63.32% and 58.11% at bagging volumes of 16，28 and 44 g，respectively，with the fastest decay at bagging volume of 28 g. The two-bank exponential decay model could better fit the relationship between the decay residue rate and the accumulated temperature during material decay，and the coefficients of determination of the equations were all greater than 0.88. The MPE，RMSE and nRMSE of the predicted and measured decay residue rates were 8%，2.47 and 7.85%，respectively. The average humification coefficients of wheat straw，hairy vetch and mixture were 54.05%，31.32% and 41.90%，respectively. Material mixing significantly increased the percentage of easy decomposition of dry matter mass（P<0.01），and the values of mixing effect increased by 13.27%，15.06% and 8.61% at bagging volumes of 16，28 and 44 g，respectively，with the highest mixing effect at bagging volume of 28 g. The material mixing significantly increased the percentage of dry matter easy to decompose，and the decomposition rate was accelerated. The mixing of wheat straw and hairy vetch accelerated the decomposition of materials， and the best effect was achieved when the bagging volume was 28 g（equivalent to 5490 and 3393 kg·hm^-2 of wheat straw and hairy vetch returned to the field，respectively）. The results could provide theoretical and technical guidance for straw return to the field in the agricultural area of eastern Qinghai.