The invention discloses a flaw precise positioning method for steel rail ultrasonic flaw detection. In the method, a steel rail to be tested is processed into a plate sample of which the thickness is the same as that of the rail waist, a detection route in which the directions of two wheels are vertical is adopted, the central point of a flaw source of the steel rail is determined, and the flaw point is detected and analyzed by a linear cutting method; and thus, an ultrasound probe is in complete fit with the test sample, the detection accuracy is improved, the hit rate and accuracy of the positioning of the central point of the flaw source are increased, detection result offset caused by the offset of the central point is avoided. In addition, the linear cutting mode is used for dissecting the flaw source of the steel rail accurately and the data on the central point of the flaw source and the flaw type can be obtained by further detection and analysis, so an accurate detection basis is provided for improving the quality of the steel rail; and by corresponding control measures, the flaw rate of the steel rail is lowered effectively and the internal quality of the steel rail is assured.