Aimed at difficulties of measuring level of the cans storing high viscidity liquids, the paper puts forward a new method to measure liquid level, whose principle is using laser to measure distance. Two signals with different frequencies superpose each other and drive a laser launcher. The laser beam with two frequencies launched at the same time was reflected back at liquid surface (tiny bead reflector needed). The reflected laser beam is received by a laser receiver and then is divided into two signals to input to two mixers. After enlarged in time scale, outputs of mixers are input to phase discriminators. After data processing, high part and low part of the liquid level with different precision can be worked out. The analysis of measuring principle of this method and experiments on the field show: the general measuring error of liquid level is less than ±2mm, the measuring range is 1~25m.