The under keel clearance control is one of the most important things for VLCC in ports, the key of determination UKL is the squat caused by vessel sailing.This paper gives a new precision method to measure the actual squat of the vessel.The principle of measurement, the selection of sensors, the integration of system and the techique of data analysis are introduced.Several examples prove the correctness, the reliability and the precision of the method.The results of the actual measurements have the important meaning for VLCC operation in ports.