Aerogels have extremely low density, high porosity and superior insulation performance; therefore they have widespread application value and prospect. But they are semi-transparent for thermal radiation at high temperature. Thermal conductivity measurement is an important way of evaluating their thermal insulation performance. The theoretical basis of transient plane source method is based on unsteady differential equation of conduction which is suitable for opaque medium. For semi-transparent materials, radiation will participate in the thermal transport process. The existence of thermal radiation within the materials will affect the test accuracy. In present study, the effect of radiative heat transfer on determining thermal conductivity of semi-transparent materials using transient plane source method is numerically studied. Semi-transparent materials with different extinction coefficient are studied at different temperature by simulating 1 dimensional steady heat transfer process the same as the experiment of steady method. The calculated thermal conductivity is selected as reference for comparison with transient plane source method. The heat transfer process of applying transient plane source method to semi-transparent materials is mimicked numerically as experimental test of transient plane source method. The results show that the thermal conductivity of semi-transparent materials measured by transient plane source method will be overestimated at temperature higher than 600 K and extinction coefficient less than 2000 m^-1 where radiative heat transfer is dominant. The deviation increases with temperature and reaches to 19.6% at 1000 K and 500 m^-1. It illustrates that radiative heat transfer will affect the thermal conductivity accuracy of semi-transparent materials.