The invention discloses an automatic line unloading device for lightweight tank bodies. A pin and a lug plate are connected through a screw to form a pin assembly, a compression spring is arranged ina pin sleeve, the pin assembly is assembled into the pin sleeve, and the pin sleeve and a supporting plate are fixed together through a connection piece; the pin sleeve is fixedly connected with a straight gear with a hub sleeve, and the straight gear is installed on a conveying chain carrying out rotary running; and press plates are symmetrically arranged at the two sides of the conveying chain,when the supporting plate arrives at a line unloading station, a front-end sensor instructs the press plates to press downwards, the minimum downward-pressing stroke is h millimeters, the press platesmake contact with and press the lug plate, the lug plate is pressed to drive the pin to move downwards by h millimeters, the top of the pin is enabled to be lower than or level with the upper surfaceof the supporting plate, a transversal push-out system arranged at one side of the conveying chain pushes out filter tanks from the supporting plate, and when the supporting plate arrives at the rearend of the line unloading station, a rear-end sensor instructs the press plates to return and move upwards until the press plates recover the original positions. According to the automatic line unloading device for the lightweight tank bodies, manual frequent operation is reduced, and the automation degree of production is increased.