Abstract: This paper reports an event of rapid evolution of relativistic electron differential fluxes in the outer radiation belt. Using Van Allen Probe-B data, we examined electron fluxes at L≈5.8, MLT≈2, and Mlat≈1.7 during the period rm 15: 19–15: 49 UT on September 7, 2016. The data show that relativistic electron pitch angle distributions transformed from butterfly-shaped to nearly flat-topped patterns within 30 minutes. Due to the slow orbital speed and minimal position changes of the satellite during this period, this observation can be regarded as an observation at a fixed spatial position. Numerical simulations indicate that the interaction with whistler-mode chorus waves is the primary mechanism for the pitch angle distribution evolution in this event.