Policy implementation fluctuation is a common phenomenon in local government. Existing studies have explained this phenomenon based on the perspective of institutional environment and section operations, but few to analyze how the changes of implementation pressure at the policy-making level affect the policy implementation. Opening this black box can help to understand policy implementation fluctuation at a deeper level. This paper takes the “physical fitness testing” reform implemented by the General Administration of Sports of China in 2019-2020 as an example, to analyze the process and mechanisms of the fluctuation at different reform stages. The study finds that policy makers shaped implementation situations with different pressure intensities through the mechanisms of mobilization, administrative control and social absorption. And implementers strategically implemented policies with self-interest motives, political tasks and policy goals as the guides of action. Further analysis reveals that: (1)Conflicting goals, insufficient incentives and constraints, as well as competition among local governments lead to negative policy implementation; (2)Under the strong incentive and hard constraint of “one-vote veto”, the unconventional means to accomplish political tasks and avoid accountability risks lead to campaign implementation; (3)Public opinion corrects the campaign-style implementation by reconciling the pressure intensity, and lead the policy to normal implementation. The above findings provide a new perspective for deciphering the deep logic of policy implementation fluctuations and throw light on the effective measures of policy implementation.