The invention provides a control device and a control method for open prevention of a vehicle three-point safety belt, and belongs to the technical field of vehicles. The device and the method solve problems in the prior art that security is not enough when children are in vehicles. The control device for open prevention of a vehicle three-point safety belt comprises a control module, and an interface sensor, a sensor with a lock, and functional keys connected with the control module, and an execution mechanism connected with the output end of the control module. The control method for open prevention of a vehicle three-point safety belt comprises the following steps: step 1, after power-on startup, selecting a processing method of lock catch for the safety belt; step 2, detecting whether a child safety seat is connected; step 3, detecting the states of the lock catch for the safety belt; step 4, after the lock catch for the safety belt is determined in a locked state in the step 3, according to the selected processing method, performing corresponding operation. The control device and the control method can prevent a child from unlocking the safety belt by the child when the child is in a vehicle.