A spring drive system for a window shade, comprising: a housing ; a first and a second gear respectively connected pivotally with the housing about a first and a second pivot axis , the first and second gears being engaged with each other, the first gear being fixedly connected with a first and a second take-up reel at two opposite sides of the first gear ; a first and a second spring 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 ; 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 ; 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 ; 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 . a housing ; a first and a second gear respectively connected pivotally with the housing about a first and a second pivot axis , the first and second gears being engaged with each other, the first gear being fixedly connected with a first and a second take-up reel at two opposite sides of the first gear ; a first and a second spring 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 ; 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 ; 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 ; 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 . 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 ; 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 . <EOS>