Based on Global Precipitation Measurement (GPM) and 40 weather stations in Tianshan Mountains, a new error correction method (MERGE) is proposed and compared with the widely used geographical different analysis (GDA) method through crossvalidation. The results indicate that both GDA and MERGE methods can improve the accuracy of Tropical Rainfall Measuring Mission (TRMM) significantly. The MERGE method not only improve the spatial resolution of TRMM, but also has higher accuracy than GDA method on annual and monthly scales. The error of corrected TRMM data varies greatly in different months. It has larger error (MAE>3 mm) in the rainy season (April-October), but less error (MAE<3 mm) in the dry season (November-March). Besides, the MERGE method can effectively improve the error in high altitude area and less depends on the original TRMM data, with the R2=0.47. So, even in local areas where the TRMM data have poor performance, this method can also reduce the error effectively.