A safety device used in combination with a child seat assembly is provided to solve the problems about complicated structure and risk for the children. The safety device includes a bracket, a first wing and a second wing pivotally connected to the head portion of the bracket, a flexible piece cooperating the first and second wings, a locking piece releasably engaged with the first and second wings. When one of the wings is pivotally rotated, both of the wings will be synchronously rotated to a vertical or horizontal state.