The invention relates to a DP-3 level dynamic positioning system and a method for guaranteeing positioning work. The system comprises a first console, a second console, a standby console, a first RCU, a second RCU, a standby RCU, three sensors, double redundancy networks, a first serial port HUB, a second serial port HUB, a master control CAN / CANOpen bus, a standby CAN / CANOpen bus, drivers, and propellers. The three consoles are respectively connected to the three RCUs through the double redundancy networks. The first RCU, the second RCU, and the three sensors are connected to the first serial port HUB. The third sensor and the standby RCU are connected to the second serial port HUB. The first RCU and the second RCU are connected to the master control CAN / CANOpen bus. The standby RCU is connected to the standby CAN / CANOpen bus. The standby CAN / CANOpen bus is connected to the master control CAN / CANOpen bus through a main / standby change over switch. The drivers are connected to the master control CAN / CANOpen bus. The propellers are connected to the drivers. Through reasonable structural arrangement, the tolerance of the DP-3 level dynamic positioning system to an external severe environment and the stability and redundancy of internal data exchange communication are fully guaranteed, and a demand of the DP-3 level dynamic positioning system from the International Marine Organization is satisfied.