The invention provides an automatic assembly device of a computer main box comprising an assembly table; two support boards are fixedly installed on two sides in upper front of the assembly table through fixing devices; through holes are provided at four front corners of each support board; the inner wall of each through hole is connected with the periphery of a sleeve through a bearing; opening ends of the sleeves face the back surface of the assembly table; a front end and a rear end of the sleeve are respectively located at a front side and a rear side of the through hole; a front end of one of the sleeves of each support board is fixedly connected with an output shaft of a motor; four sleeves on each support board are connected through a first transmission belt. According to the invention, a housing and an installing sleeve can be located in corresponding sleeves to avoid impeding the clamping of mainboards by a clamp plate; after placing the mainboard in an assigned position, thehousing and the installing sleeve can be moved out through the rotation of corresponding sleeves so that the bolt can be screwed and the mainboard can be fixed inside a cabinet and damage to the mainboard caused by direct contact is avoided.