Abstract: The article presents a general method of auto-assembly design based on entity relationship model．The ER model of mechanical system and its parts is obtained according to their characteristics．The database table structure of mechanical system and its parts is established based on the ER model．The database describes the topology and basic attributes of mechanical system，thus implementing auto-assembly design．As an example，an instance of auto-assembly of power-supply box is given，which is established based on Pro/Toolkit of Pro /Engineer．