A spring drive system for a window shade includes a housing, a plurality of rotary members, a spring and a latch. The rotary members are rotationally linked to one another and pivotally connected with the housing, wherein the rotary members include a rotary drum that is connected with a suspension member and is rotatable relative to the housing for winding or unwinding the suspension member. The spring is assembled with the housing, and can bias the rotary drum in rotation for winding the suspension member. The latch is assembled with the housing, the latch being movable to engage with and disengage from one of the rotary members to respectively prevent and allow rotation of the rotary drum.