A water flow timer comprising a main body, a water stopping membrane, a control rod, a bottom cover, a control member, a rotating member, a timing member, and a rotary cap, the timer is mechanically operated in such a way that the rotary cap is turned to actuate the control member to force the control rod to slide, thereby resulting in a gap between the water stopping membrane and a bottom opening of an inner chamber of the main body. The inner and the outer chamber of the main body are thus in communication with each other. As the rotary cap is turned, the torsion spring of the timing member is compressed to provide the rotary cap with a recovery force. The recovery of the rotary cap is delayed by an intermittent gear, so as to control the timing of the water flow.