Abstract: The law of ultrasonic wave propagation was studied according to geometrical acoustics principle while scanning on rail axle convex surface. The divergence characteristics and focusing condition of ultrasonic wave in rail axle were analyzed. Five-crystal focusing probe was designed and the probe performances were experimented. Results indicated that the focusing effect of five-crystal probe was obvious and the wave amplitude was 5 dB more than the single-crystal probe with the same size at a testing depth of 160 mm.