Target with the structural innovation of the transmission, a novel planetary worm gear transmission mechanism is proposed. The structural design and principle analysis of the novel variable speed mechanism are completed, the power transmission route of each gear (included two acceleration gears, two deceleration gears, one direct gear, one neutral gear and one reverse gear) is explained, then the transmission ratio formula is derived based on the conversion mechanism method. The three-dimensional solid model of the planetary worm gear transmission mechanism is established, and the feasibility of variable speed motion and the correctness of transmission ratio formula are verified by kinematic simulation. The mathematical model for the meshing efficiency of the planetary worm gear transmission mechanism is built, and the maximum meshing efficiency is determined to be 95% through numerical solution, which further clarifies its feasibility as a transmission.