Abstract: Abstract: Screw centrifugal pump with single blade have some features such as large wrap angle and high degree of distortion.Considering the mold making and casting reasons, semi-open design is widely adopted, which causes a gap between the edge of the impeller and the bottom cover. This gap directly affects the operating efficiency of the unit, the inside flow field, pressure fluctuation and vibration characteristics are influenced as well. In this paper, we focused on the relationship between tip clearance and external characteristics, and on the influences of the internal fluctuation. Hydraulic design and structural design were conducted aiming at semi-open screw centrifugal pump of which specific speed was 237, by means of adjusting the tip clearance and judging the current and voice, the minimum tip clearance while unit operating normally was 0.3mm, External characteristic test was conducted on the open type test bench of two stage precisions which met national standards, parameters of the best operating point were as follows, head: 9.05 m, flow rate: 196.9 m3/h, power: 6.84 kW, efficiency: 70.92%. Then the tip clearance was changed to 0.5 mm and 0.8 mm, the ratio with impeller diameter was 0.13%, 0.22% and 0.35%, respectively. External characteristic test was done separately, the influence of tip clearance to external characteristic was analyzed. Meanwhile, aiming at the design point of three kinds of tip clearance, pressure fluctuation was monitored at the edge and middle area of impeller and third section and tongue area of volute with sampling frequency was 8 192 and sampling time of each working condition was 30 - 50 s, phase reference technique was used to accomplish the separate of whole cycle's data sample. A series of pressure waveform, pressure coefficient waveform and frequency domain diagram were plotted and analyzed at the last. Results showed that when the clearance was changed from 0.3 m to 0.5 mm, head and efficiency decreased by 0.74 m and 3.6% respectively. When the clearance was changed from 0.5 m to 0.8 mm, head and efficiency decreased by 1.16 m and 5.3%, respectively. In view of pump in this paper, the allowed clearance range was 0.3 - 0.5 mm, the ratio with impeller diameter was 0.13% - 0.22%, which provided reference for the following research. Besides, at the impeller inlet area, there existed a large area of negative pressure, pressure leakage was serious when enlarging the tip clearance, thus increasing the average pressure within the cycle, which caused the pressure coefficient waveform of 0.8 mm was most gentle and 0.3 mm was most fluctuant. At the same number times of rotating frequency, the center of the impeller was larger than the inlet edge of the blade, and it was easier to capture the high frequency conversion characteristics, the main reason was that the middle radius was large and working ability was strong. In the case of 0.8 mm clearance, when pressure fluctuation was in the process of gradually increasing from the minimum value, there occurred a phenomenon of sudden drop slightly and then rose up again. The pressure wave trend at the third section and tongue of the volute was the opposite, at these two points, the amplitude of main frequency was largest when clearance was 0.3 mm, almost the same when clearance was 0.5 m and 0.8 mm. The amplitude of the main frequency at these four points was from large to small: middle area of the impeller, inlet area of the impeller, third section of the volute, tongue of the volute.