In order to analyze the influence law of injection temperature on biodiesel atomization effect accurately, using self-designed high response thermocouple, the filled trough temperature variation of the soybean fatty acid methyl ester (SFAME) were measured under the rated conditions of S195 engine. In the common rail fuel injection system, the injection temperature under engine working condition was simulated. With the Malvin particle measuring instrument, the variation of droplet diameter distribution was measured under different injection temperatures. The experimental results showed that the injection temperature was 281℃ at fuel injection beginning under the stable rated conditions of S195 engine, and presented a downtrend during the injection duration. With the control of heating device of oil supply line, the Sauter mean diameter (SMD) could be decreased by increasing the injection temperature, but increase trend of SMD was slow down with the increasing injection temperature. The preheating of biodiesel before injector will increase injection temperature in the spray process later and improve the atomization effect.