Laser powder bed fusion (L-PBF), a form of additive manufacturing technology, offers a means for the lightweight design of hydraulic fluid passages. Cavitation can lead to cavitation erosion damage and premature failure of hydraulic fluid passages. This study investigates the influence of simultaneous surface modification using secondary compensation scanning on the cavitation erosion resistance of 316L stainless steel, with a focus on the microstructure. The experimental outcomes provide evidence supporting the crucial role of simultaneous surface modification in enhancing resistance to cavitation erosion, particularly in the case of parallel modification. In vertical modification, the secondary scan with a significantly small hatch space generates a notably high energy density, resulting in the remelting of numerous previously solidified grains. Conversely, in parallel modification, the first and second scans are aligned parallelly, and the secondary scans exhibit very small hatch spaces. This configuration promotes epitaxial grain growth along the direction of heat flow, attributed to a small alpha angle, ultimately leading to the formation of large columnar grains with similar orientations. High angle grain boundaries (HAGB) possess higher grain boundary energy compared to low angle grain boundaries, indicating the presence of unstable grain boundaries. Analysis of the proportion and length of HAGBs aligns with the results obtained for cavitation erosion resistance. Furthermore, the occurrence of plastic deformation is initiated and accumulated on the grain boundaries as a consequence of the micro-jet and impact wave effects. The regions between adjacent grains become particularly susceptible to damage caused by cavitation erosion. The findings of this study carry significant implications for the optimization of L-PBF processes, as well as for enhancing the reliability and service life of hydraulic fluid passages.