The invention relates to a node fastening device, in particular to a bottom chord node fastening device of a multi-suspend structure for a building steel structure and a fastening tension method thereof. The upper end of a support rod connecting piece is a support rod transition step; a groove with downward opening is arranged in the middle of the support rod connecting piece; a cableway fixing seat is matched with and firmly fixed to the support rod connecting piece; a cable passing vertical rod is arranged in the middle of the cableway fixing seat; the upper end of the cable passing vertical rod is a downward-concave groove; the cable passing vertical rod is matched with the groove; the groove and the downward-concave groove form a cable passing pore path; and the dragging of primary and secondary cables is realized in the way of fastening and then fastening prestressing cables. The bottom chord node fastening device of multi-suspend structure and the fastening tension method thereof are reasonable in structure, high in construction precision and construction efficiency and low in construction cost, and enhance the safety of steel structure.