In order to ensure the accuracy of the split-step Fourier method used to study numerically the optical pulse evaluation in fibers, the selective problem of time window width is focused on in this paper. After giving the selective method of the time window width and the minimum time window width required for corrected calculations of optical pulse propagation in fibers, the shapes of various incident pulses after passing through different fiber lengths have been simulated by selecting different time window widths. The viewpoint that a super-Gaussian pulse distorts in shape on propagation, which was reported by other researchers, has proved to be mistaken.