The invention relates to an elongated pin horizontal auto assembling device, comprising a horizontal mounting panel, an elongated pin mounting base for a workpiece disposed above the mounting panel and an elongated pin. A pin inserting guiding unit for guiding location of the elongated pin mounting base is secured on the upper surface at the left side of the mounting panel, and an inserted pin location assembling unit for aligning the elongated pin with the elongated pin mounting base is secured on the upper surface at the right side of the mounting panel; an inserted pin catching unit is disposed on the mounting panel, and a pawl for catching the elongated pin is provided with the inserted pin catching unit; a rotating shift unit is provided at the center of the mounting panel, and is connected fixedly to the inserted pin catching unit to rotatingly shift the inserted pin catching unit so as to transport the elongated pin onto the inserted pin location assembling unit. The invented device is suitable for auto assembly of horizontal elongated pins on automated production assembly lines.