Influence of diaphragm parameter on stress amplitude and fatigue damage of diaphragm details were investigated based on Tai Zhou Yangtze River Bridge. FEM of steel bridge deck and diaphragm detail were established. Boundary conditions of the diaphragm details FEM were interpolated by sub-model method, and the accurate local stresses of diaphragm details correspond to different lateral distribution of wheels were calculated. Results show the stress amplitude of diaphragm details reach maximum when wheel off-center position is 150mm, and it decreases with the increase of wheel off-center position greatly. The stress amplitude of diaphragm details increase with the increase of the distance of diaphragms, and increasing diaphragm's thickness can improve fatigue stress of steel bridge deck.