An actuating system for a window shade, comprising: a transmission axle; a spring drive unit operable to urge the transmission axle to rotate in a first direction for raising a shading structure of the window shade; and a control module including an arrester assembled around the transmission axle, and an operating cord operatively connectable with the transmission axle, the arrester having a locking state in which the arrester acts against the spring drive unit to block a rotational displacement of the transmission axle in the first direction and an unlocking state in which rotation of the transmission axle is allowed, the operating cord being pullable to turn the arrester from the locking state to the unlocking state and to drive rotation of the transmission axle in a second direction opposite to the first direction for lowering the shading structure of the window shade; the actuating system being characterized in that: the control module further includes a cord drum connected with the operating cord, a coupling and decoupling device connected with the arrester and the cord drum, a spring connected with the cord drum and operable to cause rotation of the cord drum for winding the operating cord around the cord drum, and a release unit having an actuator operatively connected with the arrester, the actuator having an elongated shape extending along a lengthwise axis, and the operating cord extending through an interior of the actuator; wherein a pulling action on the operating cord drives the cord drum to rotate and turns the coupling and decoupling device to a coupling state, whereby rotation of the cord drum is transmitted through the coupling and decoupling device in the coupling state to drive the transmission axle to rotate in the second direction; and wherein the actuator is rotatable about the lengthwise axis to drive the arrester to switch from the locking state to the unlocking state. a transmission axle; a spring drive unit operable to urge the transmission axle to rotate in a first direction for raising a shading structure of the window shade; and a control module including an arrester assembled around the transmission axle, and an operating cord operatively connectable with the transmission axle, the arrester having a locking state in which the arrester acts against the spring drive unit to block a rotational displacement of the transmission axle in the first direction and an unlocking state in which rotation of the transmission axle is allowed, the operating cord being pullable to turn the arrester from the locking state to the unlocking state and to drive rotation of the transmission axle in a second direction opposite to the first direction for lowering the shading structure of the window shade; the actuating system being characterized in that: the control module further includes a cord drum connected with the operating cord, a coupling and decoupling device connected with the arrester and the cord drum, a spring connected with the cord drum and operable to cause rotation of the cord drum for winding the operating cord around the cord drum, and a release unit having an actuator operatively connected with the arrester, the actuator having an elongated shape extending along a lengthwise axis, and the operating cord extending through an interior of the actuator; wherein a pulling action on the operating cord drives the cord drum to rotate and turns the coupling and decoupling device to a coupling state, whereby rotation of the cord drum is transmitted through the coupling and decoupling device in the coupling state to drive the transmission axle to rotate in the second direction; and wherein the actuator is rotatable about the lengthwise axis to drive the arrester to switch from the locking state to the unlocking state. the control module further includes a cord drum connected with the operating cord, a coupling and decoupling device connected with the arrester and the cord drum, a spring connected with the cord drum and operable to cause rotation of the cord drum for winding the operating cord around the cord drum, and a release unit having an actuator operatively connected with the arrester, the actuator having an elongated shape extending along a lengthwise axis, and the operating cord extending through an interior of the actuator; wherein a pulling action on the operating cord drives the cord drum to rotate and turns the coupling and decoupling device to a coupling state, whereby rotation of the cord drum is transmitted through the coupling and decoupling device in the coupling state to drive the transmission axle to rotate in the second direction; and wherein the actuator is rotatable about the lengthwise axis to drive the arrester to switch from the locking state to the unlocking state.