Abstract: The low-noise, high-speed and high-efficiency swimming ability of marine life is unmatched by any artificial underwater vehicle. With the help of time-resolved particle image velocimetry (TR-PIV), the fine flow field measurement of the zebrafish straight acceleration swimming process was carried out, and its kinematic behaviour characteristics and dynamic mechanism were analyzed. Meanwhile, bi-orthogonal decomposition (BOD) is applied to modal decomposition of the vorticity field, and the flow field's time evolution and spatial distribution characteristics are obtained. From the perspective of flow mechanism, the flow structure characteristics and the dynamic evolution characteristics of vortices during zebrafish swimming are explored. The results showed that: The flow visualization shows the structure distribution of the overall vortex wake, which is convenient to explore the coupling relationship between the motion characteristics and the vortex wake. From the beginning of the movement, all points on the body trunk of zebrafish maintain the wavy movement law. The first few large tail swings mainly provide kinetic energy during swimming, and the subsequent tail swings mainly adjust the direction and posture. Two tail swings in different directions will form a pair of vortices in opposite directions, and the vortices will gradually fall off under the timing sequence. Meanwhile, the change of the wake vorticity reflects the change of the swimming direction of the fish to a certain extent. Based on the time evolution results after BOD decomposition, it is verified that the vorticity field in this experiment has a reasonable constant amplitude in time. The spatial distribution indicates that the low-order spatial modes characterize the main vortex structure of zebrafish swimming, and the higher-order spatial modes characterize the detailed structure of the vortex flow. The research on the tail-swinging propulsion mechanism and the dynamic characteristics of fish during swimming can provide certain scientific value for designing high-efficiency fish-like propulsion devices.