A rigid polyurethane foam auxiliary foaming agent is provided. A preparing method of the auxiliary foaming agent includes uniformly dispersing bicarbonate into a liquid component, repeatedly grindingthe mixture with a grinding device, and dehydrating the ground mixture in vacuum to obtain a finished product of the rigid polyurethane foam auxiliary foaming agent, with the water content of the finished product being not more than 1%. The auxiliary foaming agent can replace some chlorofluorocarbon type foaming agents to reduce the potential ozone consumption problem of the chlorofluorocarbon type foaming agents.