In this paper, a redundant and integrated avionics for 12U CubeSat is presented. The main innovation is to propose a method to enhance system reliability and fault tolerance based on the COTS(Commercial Off-The-Shelf) devices, as well as to deal with system recoverability for unknown failures without changing the original system hardware. A multi-node architecture based on COTS devices and standard I2C bus is designed considering low cost, modularization, simplification and high-reliability. In order to avoid the I2C bus stuck issue, a bus recovery technology generating 16 clock pulses on the I2C bus is proposed. The cold redundant strategy is utilized on the devices that have no flight heritage. The OBC employs a heterogeneous complementary dual-processor method. A high-speed processor acts the main processor for the normal command data management and attitude control tasks. A high reliability, low-speed processor works as the backup processor for monitoring and fault-tolerant operations, which can protect the system with minimum running cost when the main processor fails. The proposed avionics has been developed and successfully demonstrated on the world's first 12U CubeSat "Star of AoXiang", which was launched on June 25, 2016. the avionics worked without issues until atmospheric re-entry after 99 days in orbit. The in-orbit results of this avionics are presented in this paper.@en