By using synoptic analysis, physical diagnosis, satellite images and the Doppler radar echoes, the causes of the rainstorm process of STS Morakot is exposed. The results show that the stronger Southwest Monsoon and the incursion of the cold air both play important roles in the process of the heavy rainfall. It can be seen clearly from the characteristic of the positive vorticity at 850hPa, the ascending motion on 700hPa, and the energy gradient on the southeast of the tropical depression. And the satellite images and the Doppler radar echoes also clearly prove the rainstorm procession.