This paper describes the design of the entrance window assembly for the Full-disk Magnetograph (FMG) which is one of the three main payloads of the Advanced Space_based Sloar Observatory mission (ASO-S). The entrance window plays the role to reduce the effect of external space environment on the After Optical System(AOS), to transmit the visible light, and to prevent the infrared radiation and pollution, hence, it is one of the most important components in the design of space solar observatory payloads, as the entrance window is facing the sun all the time on orbit, both its design and verification test are very challenging task.A heat-rejecting entrance window assembly of FMG is designed to ensure that the imaging quality of the whole system is not affected by the drastic change of space environment and the temperature distributions can meet the thermal requirements, then the mechanical, optical and thermal design processes are briefly described. FMG is sun-oriented on orbit with long-term operation, and its optical system is transmissive. Thus, its mirror temperature will be more sensitive to the given solar radiation parameters than the other space optical payloads, and it is necessary to correct solar radiation parameters to improve the accuracy of simulation. Thermal analysis method with equivalent solar radiation parameters is studied to evaluate the heat-rejecting ability of the HEWA, and three analysis cases are selected based on the heat flux of the HEWA and the operational mode of FMG on orbit, while, Case1 and Case 2 are normal operational modes, and Case 3 is a calibration mode.Only 5 nm wide transmission pass-band around the science wavelength (532 nm) is able to reach the AOS of FMG because of the spectral selectivity of the window glasses. The thermal balance test of the HEWA not only needs to simulate the solar radiation intensity, but also needs to simulate solar collimation and spectral characteristics accurately. The solar simulator can simulate the solar radiation intensity, collimation and spectral characteristics adequately, it has higher heat flux simulation accuracy than other methods. Thus thermal balance test with solar simulator is carried out to verify the design and the analysis of the HEWA. Three test cases which are consistent with the analysis cases are carried out during the thermal balance test, quantitatively speaking the analysis results coincide with the test results, however, some main differences exist between them: 1) Temperatures of the baffle during the test are always lower than that of the analysis; 2) The test temperature of the window glasses in normal operational modes are always higher than that of analysis; 3) The temperature difference of Mirror1 in Case 3 is larger than that of the analysis.Some modifications made on the analysis model make the numerical analysis results being quantitatively consistent with the test results. Then the modified analysis model is used to predict the actual on-orbit temperature distribution of the HEWA. From the numerical results, it is found that only about 0.134 W solar radiation is able to pass through the HEWA and be absorbed by the primary mirror of the AOS, the maximum temperature of the window frame on-orbit is 28.2℃, and that of the window glasses is 26.3℃, while the primary mirror of the AOS is able to maintain at 22±2℃. Thus the designed HEWA of FMG is able to withstand each typical condition on orbit and meet the requirements of the mission. It avoids that the optical performance of the space solar observatory payload will decrease or the optical system will be polluted because of the overhigh temperature of the entrance window assembly and the AOS on orbit, which is able to guide the design of transmission optical system and other optical payloads for solar observation.