Lagerstroemia speciosa Zichan is a new variety of Lagerstroemia speciosa. In order to provide theoretical basis for its cultivation and promotion application, the physiological characteristics would been explored. The paper used the Li-6400 XT photosynthetic apparatus to determine the light response curves, diurnal variation and chlorophyll of biennale L. speciosa Zichan and L. speciosa sapling. The results showed that: (1) The contents of chlorophyll a (Chla), chlorophyll b (Chlb), carotenoid (Car), chlorophyll content (Chl) and chlorophyll a/b (Chla/Chlb) of L. speciosa Zichan were higher than those of L. speciosa, and there were significant differences in the above indexes. (2) Diurnal change of photosynthetic rate of L. speciosa Zichan displayed a unimodal curve, and the peak appeared at around 10:00 am, which was 17.11 μmol/(m²·s); diurnal change of stomatal conductance and transpiration rate showed unimodal curves as well. The diurnal variation of intercellular CO₂ concentration was high in the morning and evening, low at noon, and the diurnal variation of stomatal limitation value was low in the morning and evening, high at noon. (3) Diurnal change of photosynthetic rate of L. speciosa displayed a bimodal curve, and the peak appeared at around 8:00 am or 12:00 am, which was 14.41 μmol/(m²·s) and 13.16 μmol/(m²·s); diurnal change of stomatal conductance showed bimodal curve as well, and the peak appeared at around 8:00 am or 12:00 am. The diurnal variation of transpiration rate showed unimodal curve, and the peak appeared at around 10:00 am. The diurnal variation of intercellular CO₂ concentration was high in the morning and evening, low at noon, and the diurnal variation of stomatal limitation value was low in the morning and evening, high at noon. (4)Grey relational analysis showed that the order of correlation degree between P_n and environmental factors of the two materials was PAR>C_a>RH. The order of correlation between P_n and physiological factors was T_r>G_s>T_a>C_i, while G_s>T_r>T_a>C_i. (5) The light saturation point of L. speciosa Zichan was 2388.729 μmol/(m²·s) and the light compensate point was 55.590 μmol/(m²·s); The light saturation point of L. speciosa was 2080.597 μmol/(m²·s) and the light compensate point was 29.871 μmol/(m²·s). Compared with L. speciosa and L. indica, L. speciosa Zichan showed the strong photosynthetic assimilation ability, but its adaptability to low light environment was weak. L. speciosa Zichan needs plenty of light, which is suitable for planting in the region with abundant light supply.