The invention discloses a bearing press-in device of a hay mower head section assembling machine. The bearing press-in device of the hay mower head section assembling machine comprises a bearing press-in machine frame installed on the ground. Two press-in displacement guide rail installation plates are installed on the bearing press-in machine frame. The lower end faces of the two press-in displacement guide rail installation plates are each provided with a press-in displacement guide rail. The two press-in displacement guide rails are parallel to each other. A press-in displacement air cylinder installation plate is further installed on the bearing press-in machine frame. A press-in displacement air cylinder is fixed on the lower end face of the press-in displacement air cylinder installation plate through an air cylinder fixing component. The press-in displacement air cylinder and the two press-in displacement guide rails have the same moving direction. A press-in displacement plate is fixed at the end of a piston rod of the press-in displacement air cylinder through an air cylinder connector. The upper end face of the press-in displacement plate is fixed with sliding blocks of the two press-in displacement guide rails. In this way, the bearing press-in device of the hay mower head section assembling machine achieves fast and convenient feeding, high working efficiency and low production cost.