The invention discloses an irregular Venturi flowmeter, comprising a Venturi tube and at least three pressure probes, wherein the Venturi tube is composed of a front straight tube section, a contraction section, a throat section, an expansion section and a rear straight tube section; all the sections are connected in sequence; the inner diameter of each of the front straight tube section and the rear straight tube section is D; the inner diameter of the throat section is d; each of the contraction section and the expansion section is conical; the structural parameters of the Venturi tube are as follows: beta=d/D=0.45 to 0.75, the length of the throat section is 2d to 8d, the conical degree of the contraction section is 10.5 degrees, and the conical degree of the expansion section is 7.5 degrees. The invention further relates to a method for measuring gas phase flow and liquid phase flow in moisture by utilization of the irregular Venturi flowmeter. The irregular Venturi flowmeter is especially suitable for measuring downhole moisture flow, and is especially used for monitoring the gas phase flow and the liquid phase flow in an underground connected well connecting a main gas reservoir with a related gas reservoir in the process of manual migration and development of related oil-gas reservoirs in real time.