Abstract: Software OLGA is used to simulate the flow characteristics of free standing type riser system under different water depths. The calculations show that the regions where unstable flows move toward the larger flow and the magnitudes and periods of pressure fluctuations at the bottom of hybrid risers increase with the increase of water depths. Besides, the effects of the lengths and spans of flexible pipes, the flowline lengths and the riser heights on the flow characteristics of free standing type riser system are numerically investigated. With the length of flowline, the unstable flow within it is such more prone to forming. Minor changes in the lengths, or spans of flexible pipes, or the heights of risers have little influence on the flow characteristics of free standing type riser system with the same water depth.