This paper investigates the phase angles of the vibrations and hydrodynamic forces by the model testing of a flexible riser's vortex-induced vibration (VIV) under uniform flow. The VIV displacement of the riser is derived from the measured strains in the cross-flow (CF) and inline (IL) directions. Then, the hydrodynamic forces are obtained by the dynamic equation of an Euler–Bernoulli beam based on the results of VIV displacement. The characteristics of the phase angle of displacement and the hydrodynamic forces are analyzed. The results show that the phase angles of displacement and the hydrodynamic forces are almost identical at different cross sections of the riser under uniform flow. Moreover, within two adjacent vibration nodes in IL direction, the phase angle almost kept constant, while had a 180 deg change at the two sides of each vibration node. When the reduced velocity varies from 5.25 to 7.5, the phase angles of displacement derived from the flexible riser's VIV are 45 deg larger than those from the rigid cylinder's self-excited vibration.