Abstract: Primary heat exchanger is the main equipment of MJTR. As the heat transfer capacity is the main factor that affect the safety of the reactor operation, the heat exchanger need to be cleared after some time of operation. In this paper, for the total heat exchange coefficient of MJTR primary heat exchanger, the inlet and outlet water temperature of the primary and secondary cooling system are known, through calculating the logarithmic average temperature margin, heat exchange area and considering the pressure margin of the secondary cooling system, it is shown that when the differential pressure of the second side is greater than or equal to 35.15 kPa, the ability of the primary heat exchanger has descended, and the primary heat exchanger should be cleaned.