The invention provides a nut fastening device for an automatic submerged pipe flange connecting machine. The nut fastening device comprises a fastening sleeve 5 arranged on a support 1, wherein the fastening sleeve 5 is provided inside with a nut 11 and a nut pre-tightening spring 7. Two ends of the fastening sleeve 5 are fixedly connected with a front end cap 12 and a rear end cap 8 respectively.The nut pre-tightening spring 7 is in a compressed state and is positioned between a rear end face of the nut 11 and the rear end cap 8. A stop pin 9 and a spring 10 are arranged on a side wall of the fastening sleeve 5 at a position close to the front end cap 12. The rear end cap 8 is fixedly connected with a driven gear 6. The device can adapt to deepwater environment and meet operational requirements of deepwater automatic flange connecting machines.