In view of the oil leakage failure of the joint surface of the transmission case of an electric vehicle, a contact non-linear finite element method was used to analyze the tightness of the joint surface of the transmission case of an electric vehicle. Based on the workbench software, a finite element model of the transmission case is established. Determining the sealing performance of the case by calculating the distribution of the surface pressure and the opening of the case under static and high-speed conditions. Two optimization methods are proposed. The first method is to widen the box contact area of the oil leakage area; the second method is to increase the tightening bolts in the oil seepage area. After comparing the data obtained by the two optimization methods with the original data, it shows that reducing the sealing bolt spacing and widening the sealing surface where the sealing performance is poor makes the sealing performance greatly improved. This method provides an effective way to determine the tightness of the transmission case, provides a basis for optimal design, and lays the foundation for experimental research.