A novel ultrahigh-pressure large-flow vent valve comprises a seal, a valve spool and a vent pipe. The valve spool is mounted in the seal which is provided with a pressure seat inside. The pressure seat is fixed by a high-pressure flange, and the vent pipe is fixed in the high-pressure flange and communicated with an inner cavity of the pressure seat. A large-radial-clearance sealing structure is arranged between the upper portion of the valve spool and the pressure seat. The novel ultrahigh-pressure large-flow vent valve used in a steel pipe hydraulic pressure tester can save vent time of hydraulic pressure of a large-diameter steel pipe and improve production efficiency of hydraulic testing; and more revolutionarily, service life of the vent valve and reliability of sealing can be guaranteed, and weight of sealing mould of the steel pipe hydraulic pressure tester is greatly reduced.