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In order to analyze the unsteady characteristics of helical axial pump, numerical simulation of unsteady gas-liquid two phase flow in helical axial pump was carried out, and the flow field distributions of different blade heights at different flow rates were obtained. Dynamic modal decomposition (DMD) was used to decompose the transient velocity field under the condition of flow rate of 100m3/h and gas content of 30%. The first four major modes and their corresponding frequency information were obtained which can reflect the main flow characteristics. The characteristics of flow field in diffuser can be divided into dissipative modal, basic static modal and high order dynamic modal. The dissipative modal attenuated gradually with time and had little influence on the development of the flow field. The basic mode was the steady-state characteristic with a frequency of 0, which reflected the characteristics of steady state flow field caused by flow passage. The order three and order four dynamic modes reflected the rotor-stator interaction between impeller and diffuser. It gradually dissipated in the process of continuous downstream development of diffuser, and the effect of rotor-stator interaction on downstream flow of guide blade was small. The main modal field characteristics and frequency information were obtained by decomposing the phase field. The characteristics of the basic static modal field showed that on a section with a large guide blade height, the higher the velocity was, the lower the gas content was. The dynamic modal characteristics of the phase field were basically consistent with those of the velocity field. The DMD method can clearly analyze the complex transient flow characteristics in the guide vane of helical axial pump.