Abstract: We introduce an effective method to determine the emissive direction of energy bundles from a diffusing surface,which is called a tangent sphere method.The principle and implement process of the conventional method,and the tangent sphere method are analysed.It shows that the tangent sphere method is more convenient.A Monte Carlo simulation is applied to a simple model.A comparison of numerical results and integral results shows that the tangent sphere method is feasible and its precision meets engineering requirment.A comparison of computing time shows that the computational speed of the tangent sphere method is superior to that of the conventional method.