Abstract: To improve the heat efficiency of the high-temperature gas-cooled reactor intermediate heat exchanger, the theoretical calculation and simulation analysis of laminar flow and thermal boundary layers for the IHX were carried out. Through the way of polynomial velocity distribution, it is found that a third order polynomial velocity distribution is close to the Blasius exact solution and the results of the numerical simulation. At the same time, combined with the temperature distribution in different locations, it is found that upper and lower boundary layers will meet. And then, the flow changes to the Poiseuille flow and the boundary layer theory no longer applies. At last, the effects of He inlet temperature and outlet pressure were analyzed, which makes no difference. Under constant heat flux, the changes of wall temperature can be divided into obvious boundary layer zone and Poiseuille flow zone and in both areas are linear.