This paper proposed a system for the conditional monitor and fault diagnosis of tamping machine, which can collect the information of monitoring tamping machine's working condition on line in real-time. The information was processed and analyzed with an expert system, and this system can combine with the condition of monitoring railway maintenance to make reasonable judgments for the operational state of the tamping machine in order to ensure the operational quality,and can offer reliable basis for follow-up maintenance. Currently, this system has been put into operation. It improves the operating quality and speed of the tamping machine and brings better economic benefits.