Abstract: To satisfy the requirements for convenient upgrades and downloads of APP programs in practical applications, a CAN Bootloader upgrade and download system for the S32K144 microcontroller has been designed, taking into account the UDS protocol specifications. The host computer, utilizing Visual Studio as the development environment and SREC as the flashing file, communicates with the lower computer through the Kvaser USB2CAN hardware device over CAN, with service command encapsulation based on the ISO 14229 protocol framework being implemented for the download process. The transmission process has been designed in accordance with ISO 15765-2, establishing a network layer communication mechanism and error handling strategy. The microcontroller side has been partitioned into Bootloader, APP program, and simulated EEPROM flag bits. The download trigger method employs a flag bit along with a short delay, and the entire upgrade process adheres strictly to the UDS protocol specifications. Following numerous actual tests, it has been confirmed that the system is capable of achieving the complete download of the APP program. The outcomes demonstrate that the CAN Bootloader online download system accomplishes program downloads and upgrades in accordance with the UDS protocol specifications, featuring a stable and efficient upgrade process, as well as portability and scalability, thereby confirming the system’s reliability and stability.