In order to improve climate monitoring during key periods and meet the needs of meteorological services, a criterion for determination of the onset and end of the rainy season in Southwest China is investigated based on daily rainfall data from 92 stations in Southwest China during 1961-2011 and daily atmospheric circulation reanalysis data from NCEP/NCAR for 1981-2010. The results indicate that, for the mean climate, the rainy season in Southwest China begins in the third pentad in May (pentad 27) and ends in the fourth pentad in October (pentad 57). Variations in atmospheric circulation confirm the above result. Moreover, a criterion for determination of the annual onset and end of the rainy season in Southwest China is proposed based on the results of comparative analysis of different definitions and the seasonally changing characteristics of atmospheric circulation. Simultaneously, possible mechanisms influencing the onset and end of the rainy season are also discussed preliminarily.