The invention discloses a liquid level overrun detecting system and method and belongs to the field of liquid level overrun detecting. The system comprises a proximity switch, a float level meter, a guide device, a floating device and a control device, wherein the proximity switch is arranged at a liquid level overrun position, located on the outer side of an oil storage tank and connected with the control device; the float level meter is arranged on the outer side of the oil storage tank, the floating device floats on a liquid surface in the oil storage tank, and the guide device is connected with the float level meter and the floating device; when the float level meter and the proximity switch are located at the same height, the float level meter induces the proximity switch to allow the same to act and generate an on-off state signal, the proximity switch transmits the on-off state signal to the control device, and the control device determines the liquid level overrun of the oil storage tank according to the on-off state signal. By the system, the problems that liquid level overrun detecting is low in accuracy and an existing liquid level overrun detecting system is complex in testing process are solved, the liquid level overrun detecting accuracy is increased, and the testing process of the liquid level overrun detecting system is simplified.