During Goryeo-Mongol war, Goryeo naval forces defended against Mongolian military that had threatened several islands including Ganghwa Island which was temporary capital during evacuation, and protected its sea routes. Thanks to it, Goryeo could engage in prolonged war against Mongol. However, Goryeo could not withstand the Mongol’s attack which lasted for about 30 years and concluded peace treaty under unfavorable conditions. Then when it tried to return to the original capital, Gaegyeong, Sambyeolcho(three carefully selected crack troops) caused a revolt. As a number of warships and naval forces took sides with Sambyeolcho then, Goryeo naval force’s military strength significantly decreased. Before long, Goryeo naval force recovered the numbers of warships and troops to a certain degree, but they were recovered only as much as Mongol needed and according to the demands of Mongol rather than Goryeo military’s needs. Mongol, which had been greatly hindered in the execution of operation during the Goryeo-Mongol war due to the outstanding performance of Goryeo naval force, directly controlled the reconstruction of Goryeo naval force. While it was preparing for the suppression of Sambyeolcho and the first expedition to Japan, it asked Goryeo government to provide a large number of warships and naval forces to the extent that Goryeo could not afford it. It was difficult for Goryeo to meet Mongol’s excessive demand because it could not fully restore the ravages of Goryeo-Mongol war. However, Goryeo could not reject the demand of Mongol because it intended not to stimulate Mongol which was worried that Goryeo could betray, move the capital back to Ganghwa Island and resist Mongol again. Naval force was positively necessary to move the capital to Ganghwa Island and protect it. Therefore, if Goryeo did not satisfy the Mongol's demand to provide warships and naval forces, Mongol could have suspected that Goryeo would preserve its naval force's power to fight against Mongol. So Goryeo made Mongol trust Goryeo by providing warships and naval forces as required by Mongol. Since the Goryeo naval forces reconstructed by the demand of Mongol and provided to Mongol was under Mongolian control, it did not affect the strengthening of Goryeo military later on.