In order to study the reasonable thickness and height of the concrete T-beam bridge diaphragm, a 56-group concrete simple-support T beam ANSYS finite element numerical model was established, and the force performance of each group of models was analyzed with medium load and side load. The analysis results show that when the height of the diaphragm is between 54%~81% of the rib plate height, and the thickness of the diaphragm does not exceed 20 cm, the middle span deflection of the main beam, the maximum longitudinal tensile stress of the steel bar in the main beam span and the transverse tensile stress of the steel bar at the bottom of the middle plate span of the middle diaphragm have reached more suitable values. Therefore, the reasonable value of the height of the diaphragm should be between 54%~81% of the height of the rib plate, and the reasonable value of the thickness should be between 12 cm~20 cm.