The effect of transverse diaphragms on the temperature field of concrete bridge with T-girders under fire was investigated based on the character of pool fire flame spread in this study. Firstly, the dimensionless relation ship between the total flame extension length and the heat release rate was studied theoretically. Then the fire models of the T-girders bridge with 4 fire scenarios were built in the FDS to analyze thespatio-temporal distribution of the boundary temperature. Boundary temperature and heat flux were subsequently translated to the finite element model of T-girder after surface fitting in order to analyze numerically the temperature field inside the T-girders. The simulation result shows that:the proportional coefficient of 2/5 power of dimensionless heat release rate to the dimensionless total flame extension length is greater than 2.58. The diaphragm effectively reduces the boundary temperature of the indirect influence area of the fire source, and divides the boundary temperature along T-girders into four step sections with 9.7%, 41%, and 56.8% respectively in boundary temperature reduction compared to the case of no diaphragm. Besides, diaphragm reatrains the heat transmitting along the girders. In the section above fire source, the temperature girders of the T-girder are 33% and 13.3% higher in the center and bottom of the rib and 5% higher in the bottom of flange than the similar case without diaphragms.