A spring drive system for a window shade, comprising: a housing; a first rotary drum affixed with a first gear and pivotally connected with the housing, the first rotary drum being connected with a first suspension cord; a second rotary drum affixed with a second gear and pivotally connected with the housing, the second rotary drum being connected with a second suspension cord; a third gear pivotally connected with the housing, the third gear being respectively meshed with the first and second gears (124, 126); characterized in that said spring drive system further comprises: a first and a second spool (130, 132) respectively pivotally connected at two opposite sides of the third gear, the first and second spools (130, 132) being arranged coaxial to the third gear and respectively rotatable relative to the third gear; and a first spring having a first and a second end respectively anchored with the first rotary drum and the first spool, and a second spring having a third and a fourth end respectively anchored with the second rotary drum and the second spool; wherein the first and second springs (134, 136) respectively unwind from the first and second spools (130, 132) and respectively wind around the first and second rotary drums (120, 122) when the first and second rotary drums (120, 122) rotate to respectively unwind the first and second suspension cords (138, 140) therefrom, and the first and second springs (134, 136) respectively unwind from the first and second rotary drums (120, 122) and respectively wind around the first and second spools (130, 132) to drive respective rotations of the first and second rotary drums (120, 122) for respectively winding the first and second suspension cords (138, 140). a housing; a first rotary drum affixed with a first gear and pivotally connected with the housing, the first rotary drum being connected with a first suspension cord; a second rotary drum affixed with a second gear and pivotally connected with the housing, the second rotary drum being connected with a second suspension cord; a third gear pivotally connected with the housing, the third gear being respectively meshed with the first and second gears (124, 126); characterized in that said spring drive system further comprises: a first and a second spool (130, 132) respectively pivotally connected at two opposite sides of the third gear, the first and second spools (130, 132) being arranged coaxial to the third gear and respectively rotatable relative to the third gear; and a first spring having a first and a second end respectively anchored with the first rotary drum and the first spool, and a second spring having a third and a fourth end respectively anchored with the second rotary drum and the second spool; a first and a second spool (130, 132) respectively pivotally connected at two opposite sides of the third gear, the first and second spools (130, 132) being arranged coaxial to the third gear and respectively rotatable relative to the third gear; and a first spring having a first and a second end respectively anchored with the first rotary drum and the first spool, and a second spring having a third and a fourth end respectively anchored with the second rotary drum and the second spool; wherein the first and second springs (134, 136) respectively unwind from the first and second spools (130, 132) and respectively wind around the first and second rotary drums (120, 122) when the first and second rotary drums (120, 122) rotate to respectively unwind the first and second suspension cords (138, 140) therefrom, and the first and second springs (134, 136) respectively unwind from the first and second rotary drums (120, 122) and respectively wind around the first and second spools (130, 132) to drive respective rotations of the first and second rotary drums (120, 122) for respectively winding the first and second suspension cords (138, 140).