The invention relates to a safety valve for a cryogenic insulated gas cylinder, which comprises a valve body, and the valve body further comprises a gas inlet flow channel, a valve seat, a slide way, a valve core and a spring, a gas inlet connecting thread and an adjusting nut. An adjusting nut exhaust hole set on said adjusting nut, a spring set between the said adjusting nut and a spring bracket with an exhaust hole. A valve assembly further comprises the valve core, a plurality of guide rings, a positioning steel ball and a valve flap, a valve core exhaust hole is provided in the valve core. The safety valve has high stability in discharge pressure and return pressure, high repeatability and good sealing performance, and is firm in structure, difficult to be damaged and high in safety.