Abstract：Hydraulic cylinder is the first choice for driving the high speed and high power linear motion load. Aiming at the predictable motion control with high precision to the load，it must be understanded the kinematics and dynamics of the hydraulic cylinder at high running speed. An idealized hydraulic cylinder was taken as the object. The internal flow field of hydraulic cylinder and the piston movement process were simulated by using Fluent software and dynamic mesh technology，and the influence of differential pressure，the size of oil port and the working stroke on dynamic characteristics of piston were discussed. Results indicate that，the velocity of piston appeared significant fluctuations in the instants of start and end，in remaining movement process，the form of movement speed performedto be a mean value overlaid by regular high frequency pulse，a high pressure gives high pulse frequency and big amplitude of pulse；a big

size of oil mouth diameter gives high pulse frequency and low amplitude of pulse accordingly. The resultant force on piston appears fluctuations in the instants of start and end，in steady-state movement process，the resultant force performs a similar form of speed.Meantime the relationship between piston speed and the working stroke is given.

Key words：fluid power；hydraulic cylinder；dynamic mesh；characteristics of resultant force；velocity characteristics