A spring drive system (100, 100A, 100B) for a window shade, comprising: a housing; a first and a second gear (104, 106) respectively connected pivotally with the housing about a first and a second pivot axis (134, 142), the first and second gears (104, 106) being engaged with each other, the first gear being fixedly connected with a first and a second take-up reel (136, 138) at two opposite sides of the first gear; a first and a second spring (112, 114) respectively assembled at two opposite sides of the second gear around the second pivot axis, the first spring having an end anchored with the first take-up reel, and the second spring having an end anchored with the second take-up reel; a first cord drum and a third gear fixedly connected with each other and pivotally connected with the housing about a third pivot axis, the first cord drum being connected with a first suspension cord (120, 120A, 120B); and a second cord drum and a fourth gear fixedly connected with each other and pivotally connected with the housing about a fourth pivot axis, the second cord drum being connected with a second suspension cord (122, 122A, 122B); characterized in that: a first gear train is respectively engaged with the first gear and the third gear, the first gear and the third gear being respectively located at different levels along the first and third pivot axes (134, 152); and a second gear train is respectively engaged with the second gear and the fourth gear, the second gear and the fourth gear being respectively located at different levels along the second and fourth pivot axes (142, 162). a housing; a first and a second gear (104, 106) respectively connected pivotally with the housing about a first and a second pivot axis (134, 142), the first and second gears (104, 106) being engaged with each other, the first gear being fixedly connected with a first and a second take-up reel (136, 138) at two opposite sides of the first gear; a first and a second spring (112, 114) respectively assembled at two opposite sides of the second gear around the second pivot axis, the first spring having an end anchored with the first take-up reel, and the second spring having an end anchored with the second take-up reel; a first cord drum and a third gear fixedly connected with each other and pivotally connected with the housing about a third pivot axis, the first cord drum being connected with a first suspension cord (120, 120A, 120B); and a second cord drum and a fourth gear fixedly connected with each other and pivotally connected with the housing about a fourth pivot axis, the second cord drum being connected with a second suspension cord (122, 122A, 122B); characterized in that: a first gear train is respectively engaged with the first gear and the third gear, the first gear and the third gear being respectively located at different levels along the first and third pivot axes (134, 152); and a second gear train is respectively engaged with the second gear and the fourth gear, the second gear and the fourth gear being respectively located at different levels along the second and fourth pivot axes (142, 162). a first gear train is respectively engaged with the first gear and the third gear, the first gear and the third gear being respectively located at different levels along the first and third pivot axes (134, 152); and a second gear train is respectively engaged with the second gear and the fourth gear, the second gear and the fourth gear being respectively located at different levels along the second and fourth pivot axes (142, 162).