A planar internal gear enveloping drum worm (PIGEDW) reducer is designed for analyzing its contact area. In this paper, the mathematical analysis method of tooth contact is proposed to analyze its contact pattern. Firstly, the PIGEDW mathematical model is created on the basis of the gear meshing theory. Secondly, the simulation model of PIGEDW tooth surface contact is established by MATLAB. Finally, based on the previous step, a parametric study is carried out to investigate the influences of transverse module, radius of the main basic circle and inclination angle of the generating plane on the worm gear contact area. The results show that appropriate change of design parameters is conducive to improving the carrying capacity of PIGEDW. The present study offers a theoretical foundation for the subsequent enhancement of PIGEDW gear.