In order to study the applicability of the similarity law under the condition of gas-liquid two-phase flow in centrifugal pumps，the NKG65-50-140 direct-type single-stage and single-suction centrifugal pump was chosen，based on experimental and numerical simulation to study. Firstly，the performance tests of model pump under the condition of three different speeds and different inlet gas volume fractions（IGVF）were carried out at open test rig of gas-liquid two-phase flow. And then numerical simulation used the water and air as the working medium，based on the Eulerian-Eulerian inhomogeneous model，the flow condition and gas distribution were got in the impeller at different speeds and IGVF. The comparisons between the experimental and simulated results showed that the IGVF had a significant effect on the similarity law of gas-liquid two-phase flow because the IGVF affected the energy exchange capacity between impeller and liquid. For the same flow coefficient, the greater of IGVF was，the worse of the applicability to the similarity law was. With the decrease of rotational speeds，the gas distribution was moving from the tip of impeller blade to the middle of the impeller channel and the volute-tongue，which would plug the flow more easily especially when the IGVF was more than 0.03. Finally，the reliability of mathematical model and calculation methods under low IGVF was confirmed by comparing the experimental results with simulated results，which had a significant theoretical guidance for pump design.