The properties of high strength buoyancy materials for 11000m depth was studied by a full ocean depth simulation test device.The paper discussed the effects of hydrostatic pressure on water absorption and dimensional stability of the buoyancy material under the full ocean depth environment.The results showed that the water absorption was obviously influenced by the value of hydrostatic pressure, the water absorption of the material under 121 MPa was almost doubled compared with 111 MPa.Under the same pressure, the higher density material had lower water absorption.The dimensional stability of the buoyancy material was very good,which density greater than 0.70g/cm3.Its maximal size deformation rate was only 0.39% under 111 MPa and 121 MPa hydrostatic pressure.