[Objective] The influence of rainfall and temperature in semiarid loess region on runoff of Beichuan River was studied in order to provide reference for the analysis of the reasons for the change in upper reaches of Sanchuan River basin and the runoff in the middle reaches of the Yellow River. [Methods] The data from Qiandong station was analyzed by regression analysis model, accumulative anomaly model and multiple-linear regression model. [Results] Great changes existed in Beichuan drainage basin from 1989 to 2009, totally expressing an inconspicuous decreasing trend. The highest and the lowest temperature appeared in 2003, on the contrary, latent evapotranspiration presented the declining trend. The annual runoff of Beichuan river had great diversity, qualified certain degree of phased nature and mutation, showing an inconspicuous decreasing trend as a whole. [Conclusion] It is indicated that annual precipitation and temperature are the main factors influencing the runoff volume of Beichuan River, and the rainfall of flood reason plays the leading role in precipitation of this basin.