The invention provides an automatic full liquid gun stopping device for a plastic expansion water tank. The device is characterized by comprising a water tank body, wherein a main side edge seat is arranged at the left side of the water tank body; a secondary side edge seat is arranged at the right side of the water tank body; fixing assemblies are arranged on the main side edge seat and the secondary side edge seat; an intermediate separating seat core is arranged at the end part of the main side edge seat; a plurality of side connecting plate strips and connecting plate strips are fixed to the outer wall of the intermediate separating seat core; a plurality of heat dissipating holes are formed in the periphery of the main side edge seat; a tank chamber is formed in the water tank body; equipment chambers in which liquid level sensors are arranged are formed in two sides of the upper end of the water tank body a water inlet is formed in the end part of the secondary side edge seat; amini motor is arranged on the secondary side edge seat; a rotating arm is connected to the mini motor; and a sealing cover is fixedly arranged on the rotating arm. The device has the automatic full liquid gun stopping function, and is good in heat dissipating effect, convenient to mount, and stable to fix.