Asperity radius of curvature and asperity density, which are generally obtained from rough surface simulation with fast Fourier transform (FFT), are the two essential parameters for statistical contact model. In simulation, however, the value of a parameter (defined as “autocorrelation function (ACF) truncation length” in this paper), which is arbitrarily chosen and has been paid little attention to in most relevant literature, is found to have a great effect on topography parameters, regardless of the methods chosen to calculate them. Improper determination of the ACF truncation length may induce erroneous results. This paper points out how to make the proper determination of the ACF truncation length to guarantee a certain degree of precision and explains why improper determination of the ACF truncation length may cause serious errors when calculating the topography parameters. Besides, the asperity radius of curvature and the asperity density of the generated rough surfaces are calculated using the eight-summit identification method, and their formulae in terms of correlation length are obtained through numerical fitting.