The steady-state irradiation test of the fuel assembly in the test loop with high temperature and pressure is a necessary stage to reveal the anti-irradiation performance of the fuel and verify the safety of the new fuel assembly before it is put into engineering application. The influence of low parameter conditions on the heat transfer capacity of the main heat exchanger in the test loop during steady-state irradiation was studied. It is shown that the heat transfer power of two regenerative heat exchangers in parallel operation is not always higher than that of a single independent operation. When the relative flow of the primary water inlet is 40% and the temperature is 330°C, the power of two main heat exchangers operating in parallel is 3.9%~4.3% lower than that of a single unit. There is a flow turning point, and only above this flow turning point, the parallel operation of two regenerative heat exchangers is of practical significance. A method for solving the flow turning point through segmented fitting was established and the flow turning point at different primary water inlet temperatures was solved. The average difference between the flow turning point obtained by the fitting solution and the calculation solution is 0.6%, and the average deviation of heat transfer power is 1.8%. It is further proposed that the main heat exchanger can be connected in series to solve the problem of insufficient heat exchange power. When the primary water inlet temperature is 250°C, the heat exchange power in series operation is 77.5% higher than maximum power in parallel or in a single unit operation.