The realization of active modulation of reflection phase based on metasurfaces is of great significance for flexible control of electromagnetic wavefront,which makes metasurfaces have practical application values in polarization conver-sion,beam steering,metalens,etc.In this paper,a reflection phase tunable gap-surface plasmon (GSP) metasurface based on phase change materials Ge2Sb2Te5 (GST) is designed and experimentally demonstrated.By virtue of the characteristics of large permittivities difference before and after GST phase transition and the existence of stable intermediate states,the continuous modulation of near-infrared reflection phase larger than 200° has been realized.At the same time,through the reasonable design of the structure sizes,the reflection has been maintained at about 0.4 and basically does not change with the GST phase transition,which improved the working efficiency of the metasurface significantly.In addition,the coupled-mode theory (CMT) is introduced to make a full analysis of the modulation mechanism of the reflection phase,which proves that the phase transition of GST can induce the transition of metasurface working state from overcoupling mode to critical coupling mode.The improvement of the metasurface working efficiency has practical values for wavefront modulation.