The working principle and sealing mechanism of transmission seal of a new type of noncontacting end mechanical seal, the differential floating transmission seal were introduced. Based on the basic principles of fluid dynamics and static pressure lubricated bearings theory, the calculation formulas of leakage rate, pressure distribution, friction torque, end temperature, load capacity and fluid film stiffness of transmission seal were established. Numerical simulation of sealing performance of transmission seal was carried out by MATLAB software, and the effects of structure parameters and dynamic parameters on sealing performances were analyzed. The results show that the important factors influencing the leakage rate are end face clearance, pressure fluctuations and viscosity of hydraulic oil, and the ring speed has very small influence. In transmission seal, mixed friction is the main friction form, the friction torque is proportional to end surface pressure, and is similar inverse relationship with end face clearance. When the initial fluid resistance ratio is equal to 1, the fluid film stiffness takes its maximum. The fluid film stiffness is proportional to end surface pressure, and is inverse relationship with end face clearance.