To calculate the focal length of an off-axis Three Mirror Anastigmat(TMA) camera precisely and to guarantee its mapping precision,this paper researches and amends the classic mapping model and related formulas. First, the intersection angle of the off-axis TMA camera is re-defined,and the classic calculating formula for the focal length is modified. Then, the effect of earth curvature on the calculating focal length is analyzed, and the formula is further corrected. Examples indicate that when GSD is set to be 2 m,and the size of CCD, orbital altitude and the off-axis angle are 8 μm,700 km and 7°, respectively, the relative error of the focal length of a tilt-looking camera calculated by the classic formula and that calculated by the modified formula can reach to 2.6%, which will effect significantly on the mapping precision. Therefore,it suggests that the calculating formula for focal length of tilt-looking camera should take the off-axis angle into account when a off-axis TMA camera is used in photography.Furthermore, the classic calculating formula is still applicable for the ortho-looking camera.