A backrest assembly has a backrest board pivotally attached in front of a backrest frame and a buffering resilient device connected between the backrest board and the backrest frame. The buffering resilient device has two resilient brackets with through holes penetrated by a transversal adjusting rod and has a rotating tube sleeving the transversal adjusting rod to be operationally stopped by a stop bar protruded from the backrest frame. Therefore, the backrest board has a resilient and buffering efficiency relative to the backrest frame by setting the buffering resilient devices. By rotating the transversal adjusting rod to different positions, the brackets of the buffering resilient device is extended in length, and resilience of the backrest assembly is changed.