The invention relates to a high-fire-retardant glass fiber reinforced rigid polyurethane ultralow-temperature heat preserving material and a manufacturing method thereof. The material comprises 50-80 parts of common polyether polyol, 50-20 parts of fire-retardant polyether polyol, 100-130 parts of polymethylene polyphenyl polyisocyanates, 1-3 parts of surfactant, 0.1-0.5 part of catalyst, 5-10 parts of foaming agent and 5-15 parts of continous glass fiber mat. The manufacturing method includes the steps that the continous glass fiber mat is laid in a mold, other components are mixed uniformly under high-speed stirring, poured into the mold, foamed and cured to obtain the ultralow-temperature heat preserving material. The polyurethane heat preserving material can be used in the ultralow-temperature environment at the temperature of 173 DEG C, the product density is 110-140 kg/m<3>, the product thermal conductivity is smaller than 18 mw/m.K, the compressive strength is larger than 2.2 MPa, the tensile strength is larger than 3.0 MPa, the oxygen index is larger than 26, and the fire retardance can meet the standard of GB8624-2012 B2.