In order to improve the seakeeping performance of traditional planing craft in waves,this article proposes a variable structure planing craft with both speedability and seakeeping performance. The free conversion between monohull state and trimaran state can be realized by rewinding and releasing of two demihulls. In order to analyze the anti-roll effect of the demihull in oblique waves,the numerical method is used to compare the seakeeping performance of the two states of planing craft in oblique wave sailing. The results show that the demihull can reduce roll,and the anti-roll effect is most obvious in the resonance region of the monohull. When βw=60°,roll amplitude can be reduced up to 70.45%.