The invention relates to a method for measuring the wheel rail equivalent conicity by means of an under floor wheel lathe. The method is characterized by comprises following steps of enabling an axial measuring wheel of a measuring device to make contact with an A position on a wheel inside reference surface by means of the under floor wheel lathe; (2) storing the coordinate data of the wheel inside reference surface; (3) withdrawing the measuring device safely; (4) enabling a radial measuring wheel of the measuring device to make contact with an initial position B of the wheel profile; (5) scanning along the wheel profile to the wheel rim top C position continuously and uninterruptedly by means of the measuring device; (6) withdrawing the measuring device safely; (7) storing the actual geometric data of the wheel profile 4 from the B position to the C position; (8) carrying out filtering, denoising and interpolation processing to the data of the left and right wheel profiles 4; (9) leading in steel rail data by taking positions from the wheel inside reference surface 5 to the wheel rolling circle as the reference, and calculating contact points and the rolling radius difference corresponding to the transverse quantity; and (10) calculating the wheel rail equivalent conicity value.