An actuating system for a window shade includes a first rotary axle rotatable for displacing a bottom part, a second rotary axle rotatable for displacing an intermediate rail, and a limiting mechanism including a first and a second sliding part respectively linked movably to the first and second rotary axle. The first sliding part slides in a first direction when the first rotary axle rotates for lowering the bottom part and in a second direction when the first rotary axle rotates for raising the bottom part. The second sliding part slides in the first direction when the second rotary axle rotates for lowering the intermediate rail and in the second direction when the second rotary axle rotates for raising the intermediate rail. The first sliding part is prevented from sliding in the second direction via a contact between the first sliding part and the second sliding part.