<p>Strengthening and weakening of fault friction control strain energy accumulation and dissipation in the seismic cycle. Massive fluid injection leads to a dramatic change in fault friction and largely impacts the associated energy release. However, the mechanisms behind induced seismicity during continuous fluid injection remain enigmatic, and understanding the variation of major energy output is essential to assess and mitigate the induced seismic hazards. Here, we conducted laboratory fluid injection experiments on critically stressed fractures in granite and observed the inter-event creep between consecutive dynamic slip events to study the evolution of friction and energy budget. Our results demonstrate that a non-uniform distribution of fluid pressure over the fracture impedes the full recovery of frictional strength in the primary creep, promotes the extension of slipping patch in the secondary creep, and results in a transitional behavior from frictional strengthening to weakening in the tertiary creep. We found that the energy output is dominated by the strain energy for dynamic slip during the early-occurring slip events and the frictional work during the late-occurring slip events. We thus interpreted that the strain energy released during the dynamic slip event is modulated by the inter-event energy. We further explained the cumulative moment release during fluid injection at the Soultz-sous-Forêts and Pohang geothermal sites and revealed the modulation of seismic moment release during fluid injection into isolated faults, if not interacting with neighboring faults. Unveiling the variation of major energy output during continuous fluid injection provides a new revenue for assessing and mitigating injection-induced earthquakes. </p>