Aimed at the problem of oil density changing with depth in deep sea environment,the deep sea environment pressure and temperature model was established.Considering the influence of gas content,a pure oil density model and a gasliquid twophase equivalent density model were established respectively.The error analysis of the models was carried out with experimental data of VG32 and VG64 hydraulic oil.The maximum error of the models is 0.32% and 0.11%,verifying the feasibility of the models.Based on the above model,the equivalent density models with water depth as the variable were established,and the change law of hydraulic medium density in deep sea environment was studied.The results show that the change of the equivalent density of gasliquid twophase oil equivalent density model is lagged behind that of pure hydraulic oil density model with the increase of water depth.However,the difference of the density between the two models is very small when the sea water depth is more than 3 000 m,and the density of pure hydraulic oil can be used to replace the density of the two-phase medium.With the increase of water depth,the equivalent density shows a growth law from fast to slow.The specific performance is:in the shallow sea layer(<460 m),the seawater temperature plays a dominant role in the density change,and the equivalent density is increased sharply with the increase of water depth;in the middle sea layer(460~1 300 m),the temperature dominant effect is weakened,and the equivalent density is increased slowly;in the deep sea layer(>1 300 m),seawater pressure instead of temperature becomes the main factor affecting density,and density growth tends to be stable.