The gear coupling, which transmits axial compressive force, torque and relative eccentric motion, is the key component of progressing cavity (PC) pumping systems. This paper reports an innovative design of such a gear coupling especially used for submerged-motor-driven PC pumping system applied in oil fields. The novelty of this design is the synergy of a zero-tooth-difference internal meshing gear pair and a spherical pair. The principle of transmission and the determination of the key parameters such as the tangential and radial modification factors are described in this paper. Considering the reduced thickness of the gear tooth and the harsh operating condition of the PC pumping system, verification for the contacting and bending fatigue strength of the zero-tooth-difference gear pair is recommended, and the corresponding method is discussed.