Recently, Japanese government has ended the ban on exercising "collective self-defense", which indicates that Japanese policy on military force made a historic change since 1945. ANBOUND think tank scholars hold that China should have some bottom lines of Japan's expansion of military activities abroad in the future. Japan’s arms selling to Taiwan after its ending of the ban on exercising "collective self-defense" is noteworthy. China should regard this activity as a "red line." Once Japan crosses this bottom line, China must respond to it fiercely. From now on, China should start analyzing such a possibility and the way to deal with it. Sometimes, clearly showing the bottom line is a very important stance.