A kind of PDA (Personal Digital Assistant) that comprises a body and an expansion back case. The body comprises a connection groove found at its end, which connects to the body's processor. The expansion back case has default expansion functions, and it comprises a cover panel located at its end and a connection port below the cover panel. The connection port matches the connection groove when the body and the expansion back case are connected. To connect the body and the expansion back case together, simply slide the expansion back case on the back of the body until the cover panel of the expansion back case covers the connection port and the connection groove. A user may choose to carry either the binary PDA or just the body, so as to enhance the flexibility of the use of the PDA.