One of the most important subjects of overhead contact line and pantograph system is to reduce the contact loss of pantograph in high-speed operation. In this research, the relation between the wave motion of contact wire and the contact force fluctuation of pantograph is studied; the contact force fluctuation in a hanger span cycle is analyzed; and methods to reduce the contact force fluctuation are proposed. It is shown that the contact force fluctuation of pantograph in high-speed operation is mainly caused by the incident wave of contact wire that is generated by the unevenness of contact wire and is reflected at hangers. This influence becomes extremely large when the train speed is close to the wave velocity of contact wire. Increasing the wave velocity of contact wire or decreasing the reflection factor of wave motion at hangers is expected to effectively reduce the contact force fluctuation.