In this paper

a method of camera self-calibration based on the projective invariant is proposed.By the invariability of Cross Ratio in projective transform

we can compute the image of circular points via the cross ratio of circular points and vanish points of intersecting lines;With the image of circular points in three images of the planar pattern from different orientation

we can get IAC(Image of Absolute Conic)

because of the invariability of IAC in rigid motion;Using Cholesky factorization on IAC

we get internal parameters of the camera.The result of experiments shows that this method is effective and has high precision.