Abstract: Non-tethered subsurface buoy (NTSB), which combines the working modes and operational capabilities of the seabed-based buoys and Argo floats, represents one of novel marine environmental monitoring platforms. To gain a deeper understanding of the motion performance of the NTSB and evaluate its feasibility for high-precision ocean observation, computational fluid dynamic method is used to calculate the distribution of velocity field and pressure field surrounding the buoy, and the results show that the errors caused by the optimized movement of the buoy meets the requirements of national standard. After the overall structure of the buoy is determined, a six-degree-of-freedom mathematical model for underwater motion of the buoy is established and the major operating capabilities on movement depth and speed in the South China Sea is simulated by the model. Offshore experiments show that the simulation of floating and sinking motion agrees well with the measured data, suggesting that the simulation model is accurate and reliable. This study lays the foundation for the research on control methods of the fixed depth motion of the NTSB.