In the emergency case when the elevator car is over speeding in the downward direction, the traction car will be stopped rapidly by the braking of safety gear. However, the counterweight and the traction sheave are still moving, which maybe induces the collision between the counterweight and the traction sheave, the slip and off-track between the traction sheave and the rope. Therefore, a two sides mass-spring-damping rope model was proposed in this paper to investigate the dynamic behavior of elevator traction system under the braking of safety gear. In this model, the interaction between the car and the counterweight on both sides of the traction sheave was introduced. Meanwhile, the slip behavior and various constraints between the rope and traction sheave were respected in this model. Especially, the rope slack and the rope length change were considered to approach the mechanical properties of real rope. Furthermore, a numerical scheme based on Newmark- method was applied to solve the proposed dynamic model. Then the impacts of the braking force on the dynamic behavior of elevator traction system under different working conditions were deeply studied. Results showed the braking force of the safety gear, the speed and the acceleration of the traction sheave had great influences on the bounce of the counterweight. In fact, the braking performance, the vibrations of the car, rope and counterweight could be well analyzed based on this model, which is key for the realization of the steady and safe braking of traction elevator.