An offline variable monitoring method for central control units (MPUs) of an EMU train, using a monitoring system which comprises a main control unit, a serial port unit, an Ethernet interface, a TF card interface and a status indicator, wherein the serial port unit, the Ethernet interface, the TF card interface and the status indicator are all electrically connected to the main control unit; the serial port unit comprises four serial ports which are separately connected to four central control units MPUs of the EMU train; the main control unit is connected to a PC via the Ethernet interface, and a configuration software corresponding to the monitoring system is provided inside the PC; and the main control unit is connected to a TF expansion card via the TF card interface, an MAP file and a configuration file are stored in the TF expansion card, and the main control unit records monitored offline variable values into the TF expansion card; the monitoring system further comprises a FreeRTOS real-time operating system running in the main control unit, and the main control unit has a multithreaded operation; threads running in the main control unit comprise a main control thread, a serial port data monitoring thread and a TCP/IP communication thread; the threads of the main control unit are in communication with each other via a semaphore and a queue; characterized in that, the method comprises the following steps: 1) powering on the monitoring system; 2) initializing the monitoring system; 3) activating the FreeRTOS real-time operating system of the main control unit, and establishing the TCP/IP communication thread, the main control thread and the serial port data monitoring thread; 4) activating the TCP/IP communication thread, and judging whether the operating system is in a configuration mode or in a download mode currently; if the operating system is in the configuration mode or in the download mode, acquiring a configuration file or downloading an offline variable value; and, if the operating system is neither in the configuration mode nor in the download mode, returning to the step 4); 5) activating the main control thread, acquiring a current operating mode of the operating system, and judging whether the operating system is in a working mode currently; if the operating system is not in the working mode currently, returning to the step 5); and, if the operating system is in the working mode currently, reading the configuration file, loading a MAP file and an offline variable monitoring table, and setting sampling conditions for the offline variables; 6) judging whether the sampling conditions are fulfilled currently; if the sampling conditions are fulfilled, sending a sampling instruction; and, if the sampling conditions are not fulfilled, allowing the thread to enter a sleep mode, and returning to the step 6); 7) activating the serial port data monitoring thread, creating a serial port data queue, reading a serial port data, writing the serial port data into the serial port data queue, and waiting for the sampling instruction sent by the main control unit; and 8) when the sampling instruction is received, reading the serial port data queue, and judging whether the currently acquired offline variable value fulfills trigger conditions for data recording; if the trigger conditions are fulfilled, writing this offline variable value into the TF expansion card; and, if the trigger conditions are not fulfilled, returning to the step 6). 1) powering on the monitoring system; 2) initializing the monitoring system; 3) activating the FreeRTOS real-time operating system of the main control unit, and establishing the TCP/IP communication thread, the main control thread and the serial port data monitoring thread; 4) activating the TCP/IP communication thread, and judging whether the operating system is in a configuration mode or in a download mode currently; if the operating system is in the configuration mode or in the download mode, acquiring a configuration file or downloading an offline variable value; and, if the operating system is neither in the configuration mode nor in the download mode, returning to the step 4); 5) activating the main control thread, acquiring a current operating mode of the operating system, and judging whether the operating system is in a working mode currently; if the operating system is not in the working mode currently, returning to the step 5); and, if the operating system is in the working mode currently, reading the configuration file, loading a MAP file and an offline variable monitoring table, and setting sampling conditions for the offline variables; 6) judging whether the sampling conditions are fulfilled currently; if the sampling conditions are fulfilled, sending a sampling instruction; and, if the sampling conditions are not fulfilled, allowing the thread to enter a sleep mode, and returning to the step 6); 7) activating the serial port data monitoring thread, creating a serial port data queue, reading a serial port data, writing the serial port data into the serial port data queue, and waiting for the sampling instruction sent by the main control unit; and 8) when the sampling instruction is received, reading the serial port data queue, and judging whether the currently acquired offline variable value fulfills trigger conditions for data recording; if the trigger conditions are fulfilled, writing this offline variable value into the TF expansion card; and, if the trigger conditions are not fulfilled, returning to the step 6).