According to the measurement of oil film thickness on the sea, an oil film thickness sensor based on buoy is developed using vertical incidence differential laser trigonometry. The composition and principle of sensor is introduced and the process of the system integration and debugging is presented. Ceramic grade 0 gauge block is measured as a standard thickness and high order curve is fitted based on least square method for the calibration of the sensor. 1.4~9.4 mm gauge blocks are measured to verify the precision. The results show that the measurement relative error is less than 1%.