Abstract: A portable AF904 digital track circuit tester was designed to solve the problems of low maintenance efficiency, few test parameters, and unable to decode and test the digital track circuit continuously. On the basis of studying the working principle, technical parameters and field application of AF904 track circuit, the measurement,display, data decoding and analysis of basic parameters (current, voltage and frequency) of AF904 track circuit were implemented based on the key technologies of digital signal processing(DSP), such as short-time Fourier transform(DSTFT) and digital filtering. The intelligent terminal APP based on Android system was developed, which implemented the function of real-time viewing test data on mobile phones, tablets and other intelligent terminals.Combined with specially developed track trailer, it could also be tested continuously on rails. The field test results of metro lines show that the decoding accuracy of the tester is accurate, and the test accuracy of the basic parameters of track circuit can reach + 5%, which meets the daily maintenance requirements. The development of the tester solves the problem that the traditional test method is inefficient and can only test the limited parameters. It improves the maintenance efficiency of AF904 track circuit and provides data support for fault diagnosis.