In order to study the influence of gap backflow of fullflow pump impeller on hydraulic performance, the CFD software was used to carry out numerical simulation calculation of the fullflow pump. Then the flow characteristics and hydraulic performance of the fullflow pump were analyzed. Finally, the reliability of numerical calculation data was verified through model tests. There were two research directions：the first one was to study performance difference of the fullflow pump under different flow conditions when the gap was constant. The second one was to study the performance difference of the fullflow pump with different gaps under the design condition. The results showed that the gap size had an obvious impact on the hydraulic performance of fullflow pump. With the increase of gap size, the efficiency and head of the pump devices were reduced under the design condition. When the minimum gap size was 0.65mm, the head of the fullflow pump was 3.05m under the design condition, and the efficiency was 82.46%. The highest efficiency was 82.66%. When the maximum gap size was 2mm, the head of the fullflow pump was 2.92m under the design condition, and the efficiency was 8085%. The highest efficiency was 81.27%. The difference of the head between two pump devices was 0.13m under the design condition, and the difference of the efficiency was 1.61 percentage points. With the increase of the flow rate, the difference of the pressure between inlet and outlet was gradually decreased. The backflow rate was also gradually decreased. The influence of backflow on the impeller was small. And close to the rim, the effect of backflow on the flow regime was great. With the increase of the gap size, the backflow of the fullflow pump was increased. The influence of fullflow pump on flow state was heavy. The working ability of the impeller was worse. The head and efficiency of the fullflow pump were gradually decreased. The research can provide a basis for the design and development of the fullflow pump.