Spatially uniform pump distribution constitutes a fundamental requirement for high-energy laser system optimization. Especially in situations where frequency-doubled beam pumping is required, analyzing the spatial beam evolution during the frequency-doubling process is essential. This work investigates spatiotemporal evolution characteristics during second-harmonic generation (SHG) through numerical and experimental study. A laser system comprising a custom-designed regenerative amplifier (RMS ≤ 0.8%) and a double-pass Nd: YLF amplifier chain (RMS ≤ 0.7%) was employed to perform controlled SHG experiments. Through simultaneous monitoring of conversion efficiency dynamics and beam profile evolution, we demonstrate that the spatial uniformity follows deterministic transformation patterns during nonlinear frequency conversion. Notably, optimization of beam uniformity was achieved at the fundamental power density of 0.478 GW/cm² in our configuration, while maintaining conversion efficiency exceeding 85%. These findings provide valuable insights into spatiotemporal coupling mechanisms during SHG processes and offer practical significance for optimizing high-energy laser system design.