A method for downloading and updating product ECU applications was proposed. According to it, a ECU (re)programming device called BootLoader has been devised and developed. BootLoader had abilities of downloading and updating applications of ECU and can perform reprogramming of calibration data. Seed&Key algorithm was used to improve the system security. Special Format File Generator and multilevel CRC checksum mechanism are implemented in order to guarantee integrity and correctness of production target files. BootLoader has been used for diesel product ECU development process in China FAW Group Corporation. The results prove that it can meet requirements during actual development and manufacturing and have many merits such as simple structure, versatility, high reliability, low cost and easy operating etc.