The invention provides an automatic assembly device for assembling parts onto a workpiece. The automatic assembly device includes a feeding mechanism and an assembly mechanism. The feeding mechanism is used for mounting the parts onto the workpiece. The assembly mechanism includes a bracket, a moving component and an assembly component. The moving component is arranged on the bracket to drive theassembly component to move. The assembly component includes a bearing plate, a fixed frame, an assembly driving member, an assembly member and a guiding member, wherein the assembly driving member drives the assembly member to move to press and assemble the parts onto the workpiece, the assembly member is provided with a connecting portion, the guiding member is provided with a guiding portion, the guiding portion is detachably movably connected with the connecting portion to guide the movement of the assembly member, and the connecting portion can rotate around the guiding portion to automatically adjust the angle of the assembly member to press the parts adaptively. The automatic assembly device can achieve accurate assembly of the workpiece.