Based on the principle of computational fluid dynamics and the numerical simulation of the overall temperature distribution on a fully welded plate type heat exchanger,temperature distribution of superheated steam at the entrance of plates was analysed detailedly.The relationships between mass flow rate at the entrance,inlet diameter and thermal stress were investigated.It is concluded that the opening rate should be about 70% and the mass flow rate should be reduced as much as possible under the condition that technology requirements are satisfied.The results provide some reference to solve the problem of excessive thermal stress,which may lead to the destruction at the entrance of plates of a fully welded plate type heat exchanger.