Abstract: Offshore floating structures would bring the catenary risers moving in the water periodically under sea loads, then to generate relatively oscillatory flow between the riser and the water particles. Such oscillatory flow would easily trigger the "intermittent VIV" at sag-bend of the catenary riser. Experimental investigations on the behaviors of a 4m long straight flexible cylinder in the relatively oscillatory flow were carried out in this paper by forcing it oscillating with different combinations of both maximum reduced velocity U_Rmax and Keulegan-Carpenter number (KC) in still water. Fiber brag grating (FBG) strain sensors were used to measure the VIV responses of the model cylinder. Meanwhile, modal analysis was adopted to process the experimental data, and three VIV developing process in oscillatory flow including "building-up", "locking-in" and "dying-out" were firstly proposed. Furthermore, effects on VIV in oscillatory flow from both reduced velocity U_Rmax and KC were discussed and summarized. Finally, normalized time ratio contour plots of three VIV developing phases in different maximum reduced velocities were obtained.