The signal phase change caused by dispersion is transformed into nonlinear damage after square law detection, which limits the performance of C-band intensity modulated direct detection(IMDD) optical system. Traditional Volterra nonlinear equalizer (VNLE) can compensate such nonlinear damage, but its implementation complexity is very high, so it is not suitable for low-cost and short-distance transmission systems. Neighbor grouped Volterra nonlinear equalizer(NG-VNLE) greatly reduces the implementation complexity by degenerating Volterra terms with the same coefficients, but the degeneracy operation also ignores the differences between Volterra nonlinear terms and reduces the equalization performance. A low complexity nonlinear equalizer for IMDD optical system is proposed. By introducing the nonlinear term based on absolute value, the difference between the neglected Volterra term is compensated. A 107 Gb/s four-level pulse amplitude modulation (PAM4) C-band transmission of IMDD optical system based on directly modulated laser (DML) was carried out to test the performance of the proposed nonlinear equalizer. The experimental results show that it can achieve the performance similar to that of traditional VNLE, and the implementation complexity is reduced by 28%.