Entropy of the black hole

By using the method of quantum statistics, we directly derive the partition function of the bosonic and the fermionic field in a black hole and obtain the integral expression of black hole's entropy. Then via the improved brick-wall method, the membrane model, we calculate the statistical entropy of the black hole. We can choose a proper parameter to let the thickness of the film tend to zero and make it approach the surface of the horizon, Consequently the entropy of the black hole is proportional to the area of the horizon and the entropy to which the cosmic horizon surface corresponds is proportional to the area of the cosmic horizon. We also consider the influence of the spinning degeneracy of particles on the entropy. In our result. the stripped term and the divergent logarithmic term in the original brick-wall method no longer exist and any approximation is not taken. In the whole process, the physics idea is clear; calculation is simple. We offer a new simple and direct way of calculating the statistical entropy of different complicated black holes.