Generalized D-H method has been widely researched and applied in robot kinematics modeling, but there is a fundamental flaw, which is that the homogeneous transformation matrix obtained by generalized D-H method is a singular matrix, so the model we get is also incorrect, and we cannot analyze correctly. The modified D-H method is improved to avoid the defects of generalized D-H, and to ensure the accuracy of the models. In this paper, the general kinematic modeling method and the application of the D-H method and its' defects and the modified D-H method are studied. At last, the correctness of kinematic model established by the modified D-H method in 6-DOF robot is verified.