Disclosed are a flame-retardant bismaleimide resin and a preparation method thereof comprising: forming an eugenol salt suspensoid from a whole biomass eugenol in the present of a strong alkali metal base, adding dropwisely a solution of phosphorus oxychloride in methyl chloride to react to obtain a biomass-based phosphate flame-retardant; mixing bismaleimide, 2,2′-diallylbisphenol with the obtained phosphate, curing and posttreating to obtain the flame-retardant bismaleimide resin. Compared with petroleum-based bismaleimide resin, the flame-retardant bismaleimide resin prepared by the present invention is obtained from eugenol derived from biomass, and the raw material is green and renewable. It maintains mechanical properties and processing properties of the petroleum-based bismaleimide resin and has an excellent flame-retardant property. The preparation method provided by the invention has the advantages of low cost and simple production process, and has wide application prospect in the fields of aerospace, electronic information, electrical insulation and the like.