In wavelength-division multiplexing (WDM) optical fiber communication systems, the signal quality after transmission can be severely impaired by self-phase modulation (SPM), cross-phase modulation (XPM) and four wave mixing (FWM) effects. A simple formula to estimate FWM effect is derived. Furthermore, during the nonlinearity simulation in WDM systems by single-channel method, a step-size formula is proposed to optimize between computing accuracy and speed. An alternative simulation method for GVD, SPM, XPM and FWM based on multiple-channel coupled-wave equations is also developed, resulting in significant decrease in computation workload.