Abstract: The discrete ordinate approximation method of three-dimensional radiative transfer equation in absorbing scattering medium is expounded. The selection of quadrature sets of in-scattering term are analysed. The causes about the formation of false scattering and ray effect and its influence on the accuracy of solutions are also analysed. By detailed comparison, it is shown that false scattering and ray effect exact not only in discrete-ordinates method, but also in zone method, discrete transfer method, etc., and the influence of false scattering and ray effect on solution accuracy of radiative transfer equation can be reduced by decreasing grid size and increasing the number of solid angle discretation. The process of radiative heat transfer in a three-dimension rectangular furnace is simulated using discrete ordinates method. By comparison with zone method and discrete transfer method, it is found that discrete ordinates method has a good accuracy, and recently is one of the best method to simulate the process of radiative heat transfer in furnace.