The invention provides a continuous online detection device and method of a electroslag continuous casting crystallizer liquid level, and belongs to the technical field of metallurgical industry detection. The device comprises an image collector, a cloud deck, an image collector support frame, a laser power supply line, a camera data line, a liquid level height monitoring computer, a serial data line, a relay controller, an alarm lamp light control wire, an alarm lamp sound independent switch, an alarm lamp sound control wire, a sound-light alarm lamp and a 220 V power supply line. The method comprises the steps that the devices are connected and adjusted, measuring software is started, software parameters are set, a light filter is adjusted, automatic measuring is started, and the measuring device is shut down after measuring is completed. The continuous online detection device and method are used for conducting online detection on the electroslag continuous casting crystallizer liquid level height and have the advantages of being higher in accuracy of liquid level height measurement, convenient to use and low in cost.