According to the surface scattering of a mirror surface at the short wavelength band, a resolution calculation method based on the surface roughness of mirror was presented for an Extreme Ultraviolet(EUV) telescope. First, the propagation of scatter light in two mirror systems was analyzed in details, and the relationship between the distribution of light energy on focal plane and the ratio of mirror surface roughness to wavelength was discussed. The surface roughness of the mirror was measured in different wavelength bands and the one-dimensional Power Spectral Densities(PSDs)on all relative spatial frequency ranges were fitted by a k-relevant model. The numerical results show that the effective RMS surface roughnesses of primary and secondary mirrors are 0.59 nm and 0.77 nm in the spatial frequency from 1 / D (D is the diameter of the mirror)to 1 / λ(λ is an incident wavelength). The optical design software Zemax was used to build a non-sequence model of EUV telescope which contains the information about mirror surface roughness. This model can reveal the behavior of scatter light on the mirror surface and can calculate the resolution of the image plane. The results indicate that the radius of 80% energy changes from 3.9 μm to 4.3 μm and the corresponding angular resolution of the telescope is 0.25″. This method is convenient, effective and suitable for imaging evaluation.