The invention provides a dynamic monitoring device and method for the liquid level of an oil tank of a hydraulic system. The dynamic monitoring device for the liquid level of the oil tank of the hydraulic system comprises a controller and a liquid level sensor which is used for detecting the liquid level of the oil tank of the hydraulic system. An analog signal output end of the liquid level sensor is connected with an analog quantity input module of the controller through a communication cable. A digital quantity output module of the controller is connected with an alarm. The controller is connected with a display unit. The dynamic monitoring device can dynamically monitor the liquid level of the oil tank and can give an alarm in time when the oil tank begins to leak oil, so that early checking and handling are facilitated, potential safety hazards are eradicated, and economic loss is lowered; besides, the variable quantity of the liquid level of the oil tank in a preset time period can be known through calculation, so a decision-making basis can be provided for an operator, and the operator can make a correct selection between halt loss and leakage loss conveniently; moreover, the dynamic monitoring device is simple in structure and convenient to popularize and apply.