An optimal design method of a hexagonal paper core total heat exchanger for building air conditioner heating and ventilation comprises the steps that firstly, the outline dimensions of hexagonal flat paper plates are determined; then the required area and the number of channels of the paper core total heat exchanger are determined through the quadratic equation, specifically, firstly, the vapor partial pressure, the moisture contents and the specific enthalpies of a fresh air inlet and an exhausted air inlet, and the total exchanging heat quantity of the exchanger are calculated through known parameters of the fresh air inlet and the exhausted air inlet and the heat and humidity exchange efficiency required in the design, then the quadratic equation is built, the temperature of a fresh air outlet is solved, accordingly, parameters of the fresh air outlet are obtained, and then parameters of an exhausted air outlet are obtained through the fresh air and exhausted air heat balance relation; then, the heat transfer temperature difference, the mass transfer humidity difference, the heat transfer coefficient and the mass transfer coefficient of the total heat exchanger are calculated; and finally, the required area and the number of the channels of the total heat exchanger are obtained according to the heat transfer and mass transfer relational expression. The performance of the paper core total heat exchanger is promoted, the defect of iterative computation of the temperature and the humidity in an existing method is overcome, and the design method is simple and efficient.