According to the gear meshing principle, the basic parameters of external gears with straight tooth profiles were defined by reference to the parameters of involute gears. The relationships between the tooth angle, pressure angle and minimum number of teeth were obtained, and a coordinate system was established to obtain the tooth profile equation. On this basis, the meshing limit points were studied. In order to meet the requirements of continuous transmission, the meshing line of a linear meshing gear pair with a straight line tooth profile out gear was deduced, which is a curve with increasing curvature. The meshing characteristics of the transmission of the internal meshing gear with a straight line tooth profile out gear were analyzed. According to the theory of contact ratio, the formula of the contact ratio of linear conjugate meshing gears was deduced to ensure the requirement of continuous transmission when the tooth parameters were designed. According to the basic parameters of the gear and the calculation formula of the contact ratio, the relationships between the contact ratio and the height coefficient of the external gear tooth crown, the height coefficient of the ring gear tooth tip as well as the pressure angle were studied.