With the high speed railway utilization, the probability of defects or fatigue cracks in railway axles is increased. An automatic ultrasonic inspection system for railway axles is presented. This system uses combined probes and inspects the defects with spiral trajectory along the axis of the axle. Through the matrix representation of C-scan image element, a defect edge extraction method is adopted, with which the defect parameters of crack are obtained automatically. Based on these defect parameters, the stress intensity factor is assessed by svm regression and the method to predict remaining life is proposed.