The application provides a method and system for ultrasonic flaw detection of a steel rail based on automatic threshold control. The method comprises: acquiring a digital signal corresponding to a current ultrasonic flaw detection signal of a target steel rail; carrying out feature extraction on the digital signal to obtain a feature group including a damage echo height and a steel rail grass-waveheight; screening the damage echo based on a current damage echo collection threshold and generating a B type map for the damage identification of the target steel rail; on the basis of the steel rail grass-wave height, acquiring an adjustment value corresponding to the current damage echo collection threshold based on a fuzzy control algorithm; and before acquisition of a digital signal corresponding to a next ultrasonic flaw detection signal of the target steel rail, adjusting the current damage echo collection threshold automatically based on the adjustment value until completing the ultrasonic flaw detection process of the target steel rail. According to the application, the automatic threshold controlling in the ultrasonic flaw detection process of the steel rail is realized effectively, so that the timeliness, accuracy and reliability of the steel rail damage identification process are enhanced effectively.