Abstract: The net photosynthetic rate of Mi-ben Pumpkin and the effecting factors of PAR, temperature, air humidity, and CO2 content and transpiration rate were studied. The net photosynthetic rate was double-peak and exited significant photosynthesis “noon-rest” on sunny day. It was single-peak on cloudy day. The effecting factors had significant day-change Character. The solar radiation was powerful on sunny noon. The average PAR was 1246.3?mol/(m2·s) from 12:00 to 15:00. The temperature was high on sunny noon. The average air temperature was 31.7℃ and the leaf temperature was 37.4℃ from 12:00 to 15:00. The air humidity was low and was 42.3% from 12:00 to 15:00. These factors were main environment factors leading to “photosynthesis noon-rest”.