Long arm centrifuge is an important equipment to test the working performance of key equipments in the centrifugal field.The mobile balancing system which is the key component to make the centrifuge work normally and reliably is used to reduce the unbalanced force to keep the centrifuge balanced automatically and accurately.According to centrifuge working condition,the mobile balancing system which consisted of motor,synchronous belt,gear,screw nut pairs,guide rail slider and force transducer was designed.The working mechanism of the balancing system was expounded.The design parameters of the main parts were confirmed and the mechanics analyses to the key parts were done.It is shown that the mobile balancing system which is designed well can effectively improve the working conditions of the centrifuge.