The invention relates to a four-unit three-bus redundancy heterogeneous GNC system. The four-unit three-bus redundancy heterogeneous GNC system comprises a flight control computer, a sensor, an actuator and 1553B buses. The GNC system is configured with three 1553B buses to complete the transmission of navigation information and control commands to further achieve the information exchange among the flight control computer, the sensor and the actuator. The four-unit redundancy flight control computer is used for processing navigation information and guidance control commands of the whole system. The double-redundancy or triple-redundancy sensor is used for completing the information collection of the whole task section of a spacecraft. The triple-redundancy actuator is used for completing the control commands sent by the flight control computer. The four-unit three-bus redundancy heterogeneous GNC system can meet the requirements of real time, fault tolerance and reliability of the system in the whole task cycle.