An actuating system for a window shade, comprising an electric motor unit and two cord winding assemblies (132, 132A, 132B, 232, 232A, 232B); wherein each of the two cord winding assemblies (132, 132A, 132B, 232, 232A, 232B) comprises: a housing (142, 242) having a first and a second end (142A, 242A, 142B, 242B) opposite to each other, and a cavity between the first and second ends (142A, 242A, 142B, 242B); a first spool portion (144, 244) connected with a first suspension cord, and a second spool portion (146, 246) connected with a second suspension cord, the first and second spool portions (144, 244, 146, 246) being disposed in the cavity of the housing (142, 242); and a transmission axle (148, 248) rotationally coupled to the first and second spool portions (144, 244, 146, 246) and extending outside the housing (142, 242) at least at the first end (142A, 242A) thereof, the transmission axle (148, 248) and the first and second spool portions (144, 244, 146, 246) being rotatable concurrently relative to the housing (142, 242) in a first direction for respectively winding the first and second suspension cords (118, 120) around the first and second spool portions (144, 244, 146, 246), and in a second direction opposite to the first direction for respectively unwinding the first and second suspension cords (118, 120) from the first and second spool portions (144, 244, 146, 246); and wherein the two cord winding assemblies (132, 132A, 132B, 232, 232A, 232B) are respectively disposed at two opposite sides of the electric motor unit, the electric motor unit being respectively coupled to the transmission axle (148, 248) of each of the two cord winding assemblies (132, 132A, 132B, 232, 232A, 232B). wherein each of the two cord winding assemblies (132, 132A, 132B, 232, 232A, 232B) comprises: a housing (142, 242) having a first and a second end (142A, 242A, 142B, 242B) opposite to each other, and a cavity between the first and second ends (142A, 242A, 142B, 242B); a first spool portion (144, 244) connected with a first suspension cord, and a second spool portion (146, 246) connected with a second suspension cord, the first and second spool portions (144, 244, 146, 246) being disposed in the cavity of the housing (142, 242); and a transmission axle (148, 248) rotationally coupled to the first and second spool portions (144, 244, 146, 246) and extending outside the housing (142, 242) at least at the first end (142A, 242A) thereof, the transmission axle (148, 248) and the first and second spool portions (144, 244, 146, 246) being rotatable concurrently relative to the housing (142, 242) in a first direction for respectively winding the first and second suspension cords (118, 120) around the first and second spool portions (144, 244, 146, 246), and in a second direction opposite to the first direction for respectively unwinding the first and second suspension cords (118, 120) from the first and second spool portions (144, 244, 146, 246); and wherein the two cord winding assemblies (132, 132A, 132B, 232, 232A, 232B) are respectively disposed at two opposite sides of the electric motor unit, the electric motor unit being respectively coupled to the transmission axle (148, 248) of each of the two cord winding assemblies (132, 132A, 132B, 232, 232A, 232B).