An actuating system for a window shade, comprising: an axle coupling part rotatable for raising and lowering a movable rail of a window shade; a braking part and a brake coupling part connected with each other; and a lift actuating module including a spool connected with an operating part, the spool being rotatable in a winding direction to wind the operating part and in an unwinding direction to unwind the operating part; characterized in that: the braking part is adapted to apply a braking force on the brake coupling part for preventing rotation of the brake coupling part; and the actuating system comprises a clutching mechanism including two clutching parts (244, 246) movable relative to the brake coupling part and the spool to selectively couple the axle coupling part to either one of the spool and the brake coupling part, wherein the spool and the axle coupling part are concurrently rotatable relative to the brake coupling part when the axle coupling part is decoupled from the brake coupling part and coupled to the spool, and the braking force of the braking part is adapted to prevent a rotation of the axle coupling part when the axle coupling part is coupled to the brake coupling part and decoupled from the spool. an axle coupling part rotatable for raising and lowering a movable rail of a window shade; a braking part and a brake coupling part connected with each other; and a lift actuating module including a spool connected with an operating part, the spool being rotatable in a winding direction to wind the operating part and in an unwinding direction to unwind the operating part; characterized in that: the braking part is adapted to apply a braking force on the brake coupling part for preventing rotation of the brake coupling part; and the actuating system comprises a clutching mechanism including two clutching parts (244, 246) movable relative to the brake coupling part and the spool to selectively couple the axle coupling part to either one of the spool and the brake coupling part, wherein the spool and the axle coupling part are concurrently rotatable relative to the brake coupling part when the axle coupling part is decoupled from the brake coupling part and coupled to the spool, and the braking force of the braking part is adapted to prevent a rotation of the axle coupling part when the axle coupling part is coupled to the brake coupling part and decoupled from the spool. the braking part is adapted to apply a braking force on the brake coupling part for preventing rotation of the brake coupling part; and the actuating system comprises a clutching mechanism including two clutching parts (244, 246) movable relative to the brake coupling part and the spool to selectively couple the axle coupling part to either one of the spool and the brake coupling part, wherein the spool and the axle coupling part are concurrently rotatable relative to the brake coupling part when the axle coupling part is decoupled from the brake coupling part and coupled to the spool, and the braking force of the braking part is adapted to prevent a rotation of the axle coupling part when the axle coupling part is coupled to the brake coupling part and decoupled from the spool.