Abstract: The pitch angle, defined as the angle between a charged particle’s velocity vector and the ambient magnetic field, is a key parameter that governs the particle’s motion within the magnetic field. In Earth’s outer radiation belt, energetic electrons display diverse Pitch Angle Distribution (PAD) patterns. These patterns are influenced by various factors and frequently undergo changes, typically occurring over timescales ranging from several hours to several days. Investigating electron PAD variations and uncovering the underlying physical mechanisms are of significant importance for understanding the dynamic evolution of the Earth’s outer radiation belt. This paper utilizes Van Allen Probe-B data to conduct a detailed analysis of the evolution of relativistic electron PADs in the outer radiation belt during an event that occurred from 15:19 UT to 15:49 UT on 7 September 2016. During this period, the satellite was operating near its apogee, with a slow orbital speed and minimal changes in spatial position, remaining approximately at the location L≈5.8, MLT≈2 and Mlat≈1.7°. As a result, the impact of positional changes on the observational results can be considered negligible. Satellite observations revealed that relativistic (E_k ≥ 1 MeV) electron PADs transitioned from butterfly patterns to flat-top patterns during this period, within a timescale of only 30 minutes, which is significantly shorter than previously reported cases. Concurrently, intense whistler-mode chorus waves were detected in this region. Based on observational data, we calculated the chorus-driven diffusion coefficients of relativistic electrons. We then simulated the evolution of electron PADs by solving a Fokker-Planck equation. The simulation results indicate that the diffusion driven by whistler-mode chorus waves is the primary physical mechanism responsible for the transformation of the electron PADs during this event. The research presented in this paper further demonstrates the significant role of chorus waves in the evolution of the radiation belts.