Abstract: Wavefront rms gradient is a very improtant optical parameter used in ICF system. Critical technique involved in numerical computation of wavefront gradient are space processing, frequency filtering and computing algorithm of gradient et al. The simplest difference and central algorithm, least-square method and "five-point" algorithm are applied to compute rms gradient of tested wavefront. The analysis result shows that Quad-flip technique is a good choice for space processing and the filtering validty is more important for frequency filter. For original wavefront, the difference of rms gradient value of four methods is less than 0.01 λ/cm; while for 33 mm low-pass filtered wavefront , the difference is less than 0.001 λ/cm which is neglectable.