The invention provides an oil-in-water vaccine adjuvant and a preparation method thereof. According to the technical scheme, category and composition of surfactant are optimized; the stable oil-in-water adjuvant is prepared by virtue of a one-step process, so that autologous cellular and humoral immunity stimulation capacities of a vaccine are enhanced; and especially, the vaccine adjuvant also simultaneously comprises other water-based adjuvant ingredients which are effective on enhancing efficacy of the vaccine, so that attacking protection efficacy of the inactivated vaccine is further improved, and technical problems of an existing oil-in-water vaccine which is low in immunological activity and short in immunity persistence are solved. The preparation method of the nano vaccine adjuvant provided by the invention is simple in process and low in cost, and the obtained vaccine adjuvant is stable in dosage form. The prepared oil-in-water nano emulsion (the vaccine adjuvant) is long in stable period, easy for preservation, good in immunization effect, long in protection term, easy for injection and quite low in side reactions.