Laser pulse was decomposed into components of different frequencies using Fourier transform, and the numerical calculation model was built up for coupled equations of optical parametric amplification pumped by non-monochromatic light. Effects of non-monochromatic light on small signal gain, large signal gain and gain bandwidth of optical parametric amplification were investigated. Though non-monochromatic pump light decreases the gain of optical parametric process, it can improve the gain bandwidth. The larger the bandwidth of the pump light, the wider the gain bandwidth. Reasons for parametric gain decrease and gain bandwidth improvement were analyzed with respect to the phase mismatch and the parametric bandwidth.