In order to calibrate the time constant of the turbine flowmeter, a liquid step flow test device was established. The device uses a screw pump to transport fluid to form a platform flow, and an internal meshing gear pump to transport fluid to form a step flow component. The step flow component is superimposed on the platform flow by controlling the opening and closing of the high?speed solenoid valve, and the step flow is generated ultimately. The generation time of the step flow was evaluated by using a pitot tube, and the result showed that the rise time of the step flow was less than 9 ms. The time constant calibration tests of turbine flowmeter were carried out under different step conditions, and the results showed that the time constant of the turbine flowmeter was not invariable, and was greatly affected by the steady flow before the step flow, with a trend of decreasing with the increase of the steady flow before the step change, and the time constant was less affected by the step flow amplitude. In the small flow range, the dynamic flow coefficient obtained from actual measurement deviated significantly from the theoretical model derivation, while in the large flow range, the two were more consistent. Finally, the uncertainty of the calibration result of the time constant was evaluated, and the maximum expanded uncertainty was about 10 ms (k = 2). The research results are helpful for evaluating the dynamic characteristics of flowmeters and are expected to improve the accuracy of unsteady flow measurement.