The short driving range and long charging time are two big problems for electric vehicles. A concept of battery pack automatic replacement is put forward in this paper to solve these problems, and a deep research on the key techniques is contained. This paper introduced the way of positioning and locking in replacement process, including the concrete structure of both replacing equipment and battery pack. For reliability problems of the connectors, two schemes are designed. Elastic jacks and coil are adopted to guarantee the reliability and automatically centering. On this basis, battery fast replacing system is designed, which controlled by PLC, driven by electro-hydraulic servo. This was proved to be a big success in practice.