The disclosure provides a vaccine adjuvant, including a polysaccharide derived from Antrodia camphorata (also named Antrodia cinnamomea or Taiwanofungus camphoratus) fruiting body, wherein the molecular weight of the polysaccharide is greater than 100 K Da. Furthermore, the polysaccharide is obtained by an extraction process, and the extraction process includes: (a) adding powder of the Antrodia camphorata fruiting body into water to form a mixture; (b) heating the mixture under reflux; (c) after step (b), removing an insoluble matter from the mixture; (d) after step (c), adding ethanol into the mixture to perform a precipitating step and obtain a precipitate; and (e) performing an isolating step to the precipitate to obtain a fraction the molecular weight of which is greater than 100 K Da of the precipitate.