Abstract: In this study, the tomato variety "Golden Crown No.5" was used as the test material. From 2015 to 2019, the sun-shading net was used in the solar greenhouse to design different duration days (0 d, 1 d, 3 d, 5 d, 7 d, and 9 d) of less irradiation (PAR≤200 μmol·m^-2 s^-1). Blooming Date (BD), Number of Flowers (NoF), Flower Rate (FR), and Fruit Setting Rate (FSR) of each ear of tomato were measured after flowering. Also, the influence of less irradiation stress on the characteristics of flowering and fruit setting in tomatoes was analyzed, and the simulated model of the influence of less irradiation stress on the characteristics of flowering and fruit setting in tomatoes was established based on the accumulated photo-thermal effectiveness (APTE), which was verified using independent data, and compared with the simulation model constructed by PAR daily integral method (PAR). The results show that the flowering time of six panicles of tomato is delayed to different degrees under less irradiation stress, especially for the second and third panicles directly affected by less irradiation stress. The flowering time of the second panicles of tomato is delayed by about one week compared with CK for 7-9 days. The number of fruit per panicle and fruit setting rate of tomato decreases with the increase of the duration of less irradiation, and the longer the duration of less irradiation, the greater the decrease. The model based on APTE significantly improves the prediction accuracy of tomato flowering and fruit setting characteristics under less irradiation stress. Compared with PAR method, The standard errors (RMSE) of BDC, NoFC, FRC and FSRC regression estimation of the second and third panicles of tomato are reduced by 25.5%, 16.7%, 21.2%, 23.8% and 31.4%, 22.4%, 25.2%, 26.6%, respectively.