Inverse projected fringe technique is a fast and robust optical three-dimensional (3D) shape inspection technique, which is applied to online or batch inspection. In recent years, people pay more attention to the technique and apply it in many fields. A new method for generating inverse fringe is proposed. This method adopts normal mapping transform relationship to transfer coordinate. Then, it adopts the horizontal and vertical equal phase lines to obtain crosspoint which is the homologous pixel of the pixel on projector plane, so we can get the inverse fringe finally. The pruning optimization algorithm for getting the crosspoint is proposed, based on equal phase lines′ characteristic. The algorithm gradually decreases the unnecessary pixels of two equal phase lines. Finally, it can find four nearest pixels of the crosspoint, then the crosspoint can be calculated by solving two linear equations. The algorithm comes to sub-pixel level. The pruning optimization algorithm and the method proposed in Ref.[10] are used in contrast experiments. Standard phase differences in the computer simulation experiment are 0.000798 rad and 0.0046 rad respectively. Standard phase differences in the real experiment are 0.0431 rad and 0.0292 rad, respectively. The conclusion indicates that the optimization algorithm can improve precision of inverse fringe projection effectively, and reduce time consumption by exactly searching four nearest pixels with fast speed.