The invention discloses a Venturi tube flow testing device provided with sight glass and a diameter-adjustable throat pipe. The device comprises an inlet round tube section, a conical contraction section, a cylindrical throat pipe section and a conical diffusion section, wherein a first impulse tube is vertically arranged at the inlet round tube section, and a second impulse tube is vertically arranged at the cylindrical throat pipe section; an upper sliding chute and a lower sliding chute are formed in the cylindrical throat pipe section, an upper throttling slider and a lower throttling slider are inlaid in the upper sliding chute and the lower sliding chute respectively, a right threaded screw hole is formed in the upper throttling slider, a left threaded screw hole is formed in the lower throttling slider, the right threaded screw hole in the upper throttling slider is in screw thread fit with a right threaded section of a screw, and the left threaded screw hole in the lower throttling slider is in screw thread fit with a left threaded section of the screw; the screw penetrates through a measuring tube in the radial direction of the measuring tube and connected with a servo motor, the servo motor is electrically connected with a controller, and the sight glass is further arranged at the throat pipe part. According to the Venturi tube flow testing device, different drift diameters are set according to fluid with different flows, and measuring accuracy is improved.