Abstract: The complex piston serial seal system is researched by using one detailed precise 3D FSI dynamic finite element model and one direct coupling solving method based on Navier-Stokes equations. Numerically analyzed are the changes of the sealing oil film thickness, of seal contact pressure and the fluid flowing characteristics within the oil film under the conditions of different O-ring compression, and of oil-pressure and piston velocity, especially in high-velocity and high oil pressure conditions. And predicted are the sealing mechanism of oil film within the reciprocating serial seals and the reverse pumping phenomenon in high pressure and high velocity conditions. Experiments are performed to measure the sealing oil pressure and sealing gas pressure distribution along axial direction in the float piston serial seals of a hydropneumatic spring. The experimental results are consistent well with the numerical simulation ones. It shows one significant value of the 3D FE FSI model building technique and of the direct coupling computation method based on Navier-Stokes equations for the accurate design and performance prediction of complicated serial seals.