To enhance the precision and tissue preservation of medical waterjet in soft tissue surgery, this study investigated the influence mechanisms of jet parameters on cutting depth and microscopic damage. Ex vivo porcine liver experiments and Response Surface Methodology were combined to construct a multivariate regression model for cutting depth, and Scanning Electron Microscopy was employed to quantitatively evaluate tissue damage characteristics. The results indicate that the prediction accuracy of the established model exceeds 93% across different target depths. The order of influence intensity for the parameters is: jet pressure > traverse speed > standoff distance > nozzle diameter. Specifically, jet pressure and traverse speed dominate the cutting depth by overcoming tissue yield strength and regulating dwell time, respectively; meanwhile, standoff distance and nozzle diameter modulate performance by altering jet coherence. Microscopic characterization reveals that the total tissue damage area at 3 MPa is only 990 μm2, which is significantly superior to that of traditional scalpels. Furthermore, the waterjet under these specific parameters effectively removes hepatic parenchyma while preserving vascular structures with a diameter of 0.5 mm, verifying the advantages of selective cutting.