Abstract: In the case of neglecting the distortion error of the camera lens, the simplified method to calibrate cameras was deduced by establishing the proper coordinate systems, utilizing DLT mode, combining the optimization algorithm calibration method and two-step method, and decreasing the variable number to five. By the aid of a example, the parameters were solved with the tangent line of a circle, the intersection point of tangent and thecoordinates defined by the geometrical relationship of the image based on two propositions proposed in the paper. The numerical example shows that the method has less computation and high accuracy.