Acquiring the thickness of slag layer in continuous casting tundish accurately is beneficial to continous casting stably, and improves enterprise benefit. A new method to measure the thickness of slag layer in continuous casting tundish, based on temperature information is put forward in the papers. A measuring bar, made of Refractory Material, was inserted into the tundish to perceive the temperature. Then the thickness of slag layer may be obtained accurately by temperature interface between air layer and slag layer, and slag layer and molten steel layer. After applying to the slag layer thickness measurement in steel metallurgy field, it has a favorable application prospects because of the measuring error about less than 2.6mm.