Adjustment System and Method Thereof Between a Child Car Safety Seat and a Backrest

An adjustment system for adjusting relative position between a car safety seat and an onboard backrest is introduced and has a controlling unit to receive a first monitoring signal or a second monitoring signal in response to pressure change, pull change or displacement. The adjustment system allows a backrest of the car safety seat to closely lean against the onboard backrest so that the child sitting on the car safety seat is well protected when in a sudden brake or a collision.