Projective invariants are independent of viewpoints, intrinsic and extrinsic camera parameters and common used in computer vision gradually. The stability of the projective invariants is very important for the construction of the model library. It is related to the quality of the images, the precision of the feature extractor and the numerical computation. In this paper, cross ratios of the four collinear points and five coplanar points were analyzed first, then some affecting factors were concluded, and some constraints were proposed on constructing model-based object recognition system. The experiments show that the stability of the projective invariants designed under these constrains is improved highly.