This work considers the periodic gait planning of a 3‐D bipedal robot with two underactuated DOFs. Instead of planning a periodic gait by a traditional direct parameter optimization search, which is usually time‐consuming, we present a novel modification‐based periodic motion planning method, which consists of two steps. Firstly, an initial gait is obtained by relaxing the periodicity constraints in the optimization search. Then, the initial gait is modified to be a periodic one. To achieve a successful gait modification, the gait modification theory is firstly presented, and a gait modification strategy is then proposed. Finally, the presented method is illustrated on the underactuated 3‐D biped, and a comparative simulation study is provided. It is shown that, compared with the traditional method, the time cost in the presented method is significantly reduced.