Abstract: In order to research the harm of sub-high temperature in daytime on tomato (Lycopersic esculentum Mill.) and to supply scientific evidence for temperature regulation of tomato production in greenhouse, the effects of sub-high temperature in daytime (35℃) from different stages on the photosynthesis and yield of tomato in greenhouse were studied. The treatments were: the feasible temperature of 25℃ in daytime considered as the control (CK), 35 sub-high temperature in daytime treating from the 1st bud of tomato effloresced considered as T1, and from 10 days after effloresced considered as T2. The results showed as follows, a similar trend was observed for the diurnal change of the net photosynthesis rate (Pn) among all treatments，but the values of Pn in T1 and T2 obviously decreased compared with CK. The Pn of leaves at different positions in T1 and T2 decreased during the growth stages of tomato plants，and the earlier the tomato plants were treated by sub-high temperature，the lower the value of Pn was. The stomatal conductance（Gs）increased，and the limit of stomatal（Ls）decreased in T1 and T2 compared with CK. These results suggest that the decrease of Pn under sub-high temperature was maybe mainly induced by non-stoma limiting factors，and the earlier the plants were treated by sub-high temperature after they effloresced，the worse the photosynthesis the plants were. The yields were decreased because of the low photosynthesis，and the yields of T1 and T2 were decreased by 65.43% and 42.31%. There was a relationship between the low photosynthesis and the yield decrease.