Abstract: The heat loss was often neglected in existing analysis of liquid metal natural circulation loop，which may cause differences between calculation results and experimental data. In order to study the influence of heat loss on the steady-state characteristics of liquid metal natural circulation loop, a leadbismuth natural circulation loop code was developed by using MATLAB/Simulink and validated by the experimental data. Furthermore, the influences of heat loss on the steady-state parameters of natural circulation loop were analyzed by using the code under different thermal powers, secondary flow rates of intermediate heat exchanger and ambient temperatures. It is found that the natural circulation flow rate could increase by decreasing the heat loss. The flow rate would be more easily affected by the heat loss when the secondary flow rate is low. On the other hand, the ratios of heat loss to total thermal power could decrease by increasing the secondary flow rate or thermal power properly, so the heat utilization factor could also increase. The ambient temperature would not influence the loop natural circulation flow rate obviously under different thermal powers when the secondary flow rate is constant, while the heat utilization factor would increase with the ambient temperature.