In order to improve spatial efficiency and convenience in storage, transportation and launching of the catamaran, a planar linkages based foldable retractionextension mechanism was designed. Aiming at minimizing the hull angular displacement during the retraction process and the constraints on linkage mechanism condition and dimension design requirement, a dimension optimization model was established．By using the closed loop vector method, the optimized kinematic variation law of the mechanism was analyzed. The results show that the mechanism can realize the functions of retraction-extension of catamaran. After size optimization, the maximum angular displacement amplitude of pontoon is reduced by 79%, and the retraction-extension process is more stable with less impact.