An experimental device and method for co-existence of overflow and lost circulation in a fractured formation during drilling of a deviated well are provided. An inner cavity of an outer pipe is provided with an inner pipe to form a deviated casing. An upper side of the outer pipe is provided with a mixture outlet. A middle-lower side of the outer pipe is provided with a leakage port and two overflow ports. The gap width can be changed by adjusting a bolt and a support block to be compatible with different types of fracture models. A gas injection system includes an air compressor and a gas flowmeter. An upper port of a liquid injection system is connected to a joint arranged at upper ends of the inner pipe and outer pipe through a water pump, a liquid flowmeter and a liquid regulating valve.