The invention discloses a serial port upgrading method and system for multiple MCU slave stations, and belongs to the technical field of MCU upgrading, and the method comprises the following steps: all MCU slave stations are initialized to set a serial port analysis chip as read enabling, only after an upgrading frame of a corresponding slave station is obtained from an upper computer, the serial port analysis chip is set as write enabling when a reply frame is sent, and the serial port analysis chip is reset as read enabling immediately after sending is completed; therefore, the bus multiplexed by the plurality of MCU slave stations can complete data receiving and transmitting without generating conflicts; specifically, after a single MCU slave station obtains a complete upgrading file frame by frame, the MCU slave station copies the upgrading file to an application engineering storage area. By utilizing the method, a plurality of MCU slave stations can be simultaneously upgraded through a single serial port, all MCUs have the same priority in the system, the transmission speed is relatively high, the upgrading efficiency is high, the number of interfaces is not limited, the upgrading of other MCUs is not influenced when the upgrading of a single MCU fails, the stability is high, data conflicts can be avoided, and the upgrading risk is favorably reduced.