The invention provides a preparation method of an agaricus bisporus low-salt soybean sauce. The preparation method comprises the following steps of putting soybeans after being subjected to cleaning and impurity removal into room-temperature water, performing soaking, and performing draining; cooking the drained soybeans until the soybeans are completely cooked; sufficiently and uniformly stirringheated flour with the cooked soybeans when the flour is hot to obtain mixed materials; after the mixed materials are cooled, performing inoculation with aspergillus oryzae mother starters, and performing culturing until starter materials are yellowgreen so as to obtain finished starters; and taking agaricus bisporus powder or an extraction solution thereof, mixing the taken agaricus bisporus powder or the extraction solution thereof with the finished starters to obtain a mixture, putting the mixture into a fermenter for fermentation, wherein the fermentation period is 7-10 days, then adding microzyme and lactic acid bacteria, setting the temperature to be 35-40 DEG C, and continuing performing fermentation for 5-7 days so as to obtain the agaricus bisporus low-salt soybean sauce. According to the preparation method disclosed by the invention, a low-salt solid state fermentation method is adopted, the fermentation time is short, the fermentation temperature is high, growth of infectious microbes is not facilitated, the agaricus bisporus low-salt soybean sauce is safe to eat, the soybean sauce produced by the method has the advantages of being healthy, environmentally-friendly, lowin cost and short in production cycle, besides, the soybean sauce inherits the advantages of the agaricus bisporus, and the soybean sauce obtains favor of consumers.