Abstract: The change of photosynthetic physiology index at different growth period in different rice cultivars were investigated for providing a reference for breeding and cultivating rice cultivars. The photosynthetic physiology parameters of 4 rice cultivars (Combinations) of Mianyang were measured by a portable gas exchange fluorescence system. The results showed that chlorophyll content, net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), intracellular concentration of CO2 (Ci) in leaves of 4 cultivars were different and had obvious differences. The change of Chl.a/b and Pn at different growth period in 4 rice cultivars showed single peak curves.‘Chengnuo88’,‘B6You4761’and‘233A/Mianhui768’had high Tr at heading day, while‘Gangyou725’had low Tr.‘Gangyou725’had high Pn during grain filling. All those indicated that‘Gangyou725’had lower demand of water supply at heading day and higher potential productivity during grain filling than the other three cultivars.