Statistical entropy of scalar field outside Reissner-Nordström-de Sitter black hole is computed by the equation of state density corrected by the generalized uncertainty principle and by Wentzel-Kramers-Brillouin approximation method. The result shows that the entropy is proportional to the sum of the internal, the external and the cosmological horizon areas, which accords with these calculated by other methods. It shows internal relation between the entropy of black hole and horizon area. The entropy of black hole is the entropy of quantum state on horizon, which is a quantum effect.