Abstract: In light of the long term continuous automatic synchronous observations of global solar radiation (Q) and the photosynthetically active radiation (QPAR) during the growth seasons in Chengdu, Kuming and Guangzhou, it is found that the QPAR/Q ratio based on hourly accumulative radiation is almost a constant and that the season, cloud amount and diurnal variation of sun’s altitude have not any obvious influence on the ratio. This result is consistent with the previous research conclusion drawn from that in the North China Plain. It seems that the ratio would increase with the latitude getting low