The invention discloses a design method of an additional layer of an old asphalt pavement, relates to a design method, in particular to a design method of an additional layer of an old asphalt pavement, and aims at solving the problems that an existing design method of the additional layer of the old asphalt pavement is not specific in design reference, simple in design flow and relatively poor in design effect. The design method comprises the following steps: 1 determining design parameters of the additional layer of the asphalt pavement; 2 determining an asphalt pavement usability model; 3 determining an asphalt pavement transformation structure; and 4 paving the oil asphalt pavement again. The design method is applied to the field of bridge construction.