Designing methods of null test computer generated hologram (CGH) for freeform surface optical element are researched. Bicubic non-uniform B-spline interpolation is used to fit a continuous phase function from scattered phase compensation data,which is calculated by universal ray tracing numerical method. Matrix representation of bicubic B-spline and monotonicity of CGH phase function are used to calculate the precise intersection lines of planes paralleling to x-y plane and the interpolated B-spline surface,and the positions of fringes of a phase-only CGH are solved with high accuracy. Finally a design example of null test CGH for freeform surface optical element is presented.