Piston type valve cores are widely adopted in check valves, globe valves, pressure reducing valves and so forth. With the irregular inlet flow channel, the bottom of piston type valve core exits unbalanced torques usually. The unbalanced torque causes the valve core to squeeze sealing elements and increases the energy consumption. To analyze the unbalanced torque on the bottom of the piston type valve core, a transient numerical model of the piston type valve core is established and the simulation results are compared with the theoretical formula for the purpose of validation. The dynamic flow characteristics are studied to investigate pressure characteristics on the bottom of the piston type valve core with the dynamic motion of the valve core. Meanwhile, pressure characteristics under different valve core displacements are obtained, and the sensitive range of the unbalanced torque is found. This work can be helpful for the further structural optimization on the valves with piston type valve cores.