Abstract: Whipstock is essential for directional drilling with coiled tubing, and the electric whipstock offers great technical advantages with its ability to make continuous and accurate tool face angle adjustments. However, due to narrow downhole radial space and high torque requirements for directional drilling, the design of the key structure in the electric whipstock, namely the gear reducer, becomes a difficult point of development. Therefore, a kind of electric whipstock based on a high-torque motor and multi-stage planetary gear reducer was designed, and the planetary gear reducer was optimized. The product of three key parameters, i.e, bending fatigue strength safety factor, contact fatigue strength safety factor, and output torque, was used as the basis for optimization so that the optimal gear ratio was obtained under the required space, and the finite-element strength calibration and the actual load carrying capacity of the gear reducer tests were carried out. The results show that the actual load bearing capacity of the designed two-stage planetary gear reducer of the electric whipstock exceeds the design target of 800 N·m, and the instantaneous capacity can reach 1 260 N·m, which meets the requirements of directional drilling operation of the coiled tubing drilling in the field. The research results can provide a reference for the design and processing assembly of future electric whipstock.