The utility model discloses a vehicle-mounted separated fixing device for an automatic rifle. The device comprises a bracket and a grip holder which are in a separated structure, wherein a shoulder rest of the rifle is inserted into the bracket, and a grip of the rifle is clamped in the grip holder. The device has the advantage of high fixing stability and can rapidly unlock and take the rifle through a buckle assembly on the grip holder, thereby effectively solving the two mutual contradiction problems of stable fixing and rapid unlocking of a conventional rifle fixing device; meanwhile, different models and sizes of rifles can be fixed by means of the separated structure, and the device can be randomly combined and mounted according to available space; besides, the device is made of aluminum alloys, and the strength is guaranteed while the weight of the fixing device is greatly reduced.