The invention provides an inner and outer tube type flowmeter and a flow detection method. The inner and outer tube type flowmeter is characterized in that an inner tube as long as an outer tube is arranged in the outer tube, and the inner tube comprises an inlet tube, a shrinking tube, a pressure tapping tube, an expanding tube and an outlet tube that are connected in sequence. Compared with the traditional inner and outer tube differential flowmeters, the inner and outer tube type flowmeter is provided with the inlet tube and the shrinking tube at the front end of the pressure tapping tube, a fluid entering the inner tube experiences shrinking, accelerating, expanding and decelerating, and a fluid entering a gap between the inner and outer tubes experiences expanding, decelerating, shrinking and accelerating. A difference between inner and outer tube fluid speeds is maximum, to be specific, pressure tapping is performed at the middle tube with maximum pressure difference, and a maximum differential value can be acquired. The inner and outer tube type flowmeter is simple in structure and reasonable in design, the differential pressure value of the fluids in the inner tube and outer tube is increased, the sensitivity, accuracy and energy efficiency of the inner and outer tube type flowmeter are enhanced, and final measurement results are accurate and reliable.