Along with the rapid development of China's railway, express locomotive has gradually become the backbone of railway transportation. The flaw-detecting test for axles of express locomotive directly affects on the safety of express railway transportation. The digitized ultrasonic testing system is designed against the flaw-detecting test for axles of express locomotive. This paper will focus on the study of key technical problems in design, including ultrasonic emission, receiving module, express A/D acquisition model and upper computer signal-processing software. The stability and reliability of such system is tested by test of artificial defects which have the capacity to satisfy the locomotive axle inspection accuracy and adapt to test all kinds of express locomotive axle.