The invention discloses a robot tool coordinate system calibration method based on planar constraints. The robot tool coordinate system calibration method comprises the steps of: setting a motion modeof a robot; establishing a base coordinate system B, an end coordinate system E and a tool coordinate system T; establishing a planar constraint condition; ensuring that an irradiation point is always on a marble platform, keeping the attitude of the end coordinate system E unchanged, and controlling the position of the end coordinate system E to change for six times; ensuring that the irradiation point is always on a marble platform, keeping the position of the end coordinate system E unchanged, and controlling the attitude of the end coordinate system E to change for three times; and solving an irradiation point coordinate equation. During the tool coordinate system calibration process, the calibration of the tool coordinate system can be completed by solving the irradiation point coordinate equation of a laser displacement sensor, the solving of the irradiation point coordinate equation only needs to complete the measurement of 9 sets of data, the calibration efficiency is high, and the laser displacement sensor is utilized for distance detection to ensure sufficient calibration precision of the tool coordinate system.