As the country with the largest military spending in the world, the sheer size of US military budget makes its process and management extremely complicated. The complexity has brought a variety of issues, which accordingly gives rise to the need of reform. However, due to numerous factors internally and externally, the progress of the reform is slow at best. Facing a more and more severe budget deficit and militarization of foreign policy, the need for reform has not been more urgent. This article systematically analyzes the problems facing the US defense budget making and implementation, identifying factors influencing the reform, such as international system, domestic political system and personal preferences of leaders. It also evaluates the progress and trends of the defense spending reform. Trump administration might launch significant changes in its national defense strategy, but these changes must accompany reforms in military budget accordingly. In order to monitor the changes in US national defense strategy, we should closely watch the reforms in military spending.