In simulation of focused beam propagation with arbitrary focal length, the traditional beam propagation algorithm based on Fast Fourier Transform will fail to give right results due to the limitation of the sampling theorem. A new algorithm is given, in which the factor of spherical wave front is removing explicitly. Thus the propagation of a focused beam could be evaluated indirectly from propagation of an unfocused beam. The results in two cases are connected inherently by Fresnel number, which plays a key role in focused beam propagation.