An actuating system for a window shade includes a suspension member, a casing having a fixed protrusion, a transmission axle disposed through the casing, a rotary drum arranged in the casing and rotationally coupled with the transmission axle, and an impeding part connected with the rotary drum and affixed with an end of the suspension member. The rotary drum is rotatable in a first direction for winding the suspension member, and in a second direction for unwinding the suspension member. The impeding part is movable relative to the rotary drum between a first and a second position, the impeding part when in the first position being movable with the rotary drum past the protrusion in any of the first and second direction, and the impeding part when in the second position being engageable with the protrusion to block rotation of the rotary drum in the second direction.