The finite difference time domain (FDTD) method is used to simulate light propagating from vacuum to glass material covered with nano-porous films, and the transmittance spectrum with different wavelength are obtained. Compared with theoretical spectrum, the refractive index of the nano-porous film is calculated. A series of nano-porous films with the porous ratio of 5%、10%、15%、20% as well as 30% are simulated. In the end of this paper, a broad-band antireflection coating composed of the nano-porous film and one MgF2 layer is proposed. Simulated this antireflection coating by FDTD method, the average transmittance can be reached above 99.5% between 400nm and 800nm.