Gap seal is a noncontacting seal structure，not only can realize selflubrication，but also help to reduce friction force，and greatly improve the dynamic characteristic of the hydraulic cylinder．UFD was used to simulate the velocity, friction of different structure to obtain and the relationship between the velocity, friction and the piston speed/gap size. The results show that the viscous force of the cylinder surface is directly proportional to the velocity of piston speed, but viscous force of the piston surface is inversely proportional to the piston speed. The viscous force of both are equal and proportional to the gap size when the piston is static. The viscous force of both are increased along with piston speed increase, but the viscous force of cylinder surface is higher than the viscous force of the piston surface when piston moves.