Multi-MCU system architecture is often needed in applications that require parallel completion of multiple tasks or matrix control applications. When multiple Flash-less MCUs are used in the system, in order to make the system start normally, the general practice is to configure the special program Flash for each MCU to start, which not only increases the hardware cost, but also brings challenges to the mass production of products and subsequent program upgrades. In this paper, we introduce how to use MCU's I2C ISP to realize multiple MCU to share the same Flash to boot, and describe the implementation process and difficulties in detail. The results show that the method of multiple flash-less MCUS sharing a single Flash boot is feasible in practical application.