To reveal the local damage mechanism of natural gas pipelines subjected to high-velocity projectile penetration, a unified solution for the plastic radius of pipeline damage was established based on the unified strength theory, integrating penetration tests, numerical simulations, and theoretical analysis. Through projectile penetration tests on L415M pipeline steel, key parameters including impact feature on the impacted surface of the pipeline, plastic zone and plastic radius were obtained. Based on the experimental results and ANSYS/Workbench, a dynamic model was developed to numerically simulate the distribution of local stress fields and strains in the pipeline. Sensitivity analysis of the intermediate principal stress parameter $b$ was conducted using unified strength theory. Furthermore, in conjunction with a finite cylindrical cavity expansion model, an analytical expression for the plastic radius of pipeline damage was derived, and a failure criterion for local damage of natural gas pipelines under projectile penetration was proposed. According to the criterion, when the plastic radius measured under penetration loading exceeds the critical value $r_{max}$ defined by the uniaxial tensile fracture strain ${\epsilon }_f$ of the material and the model parameter $A$ (which incorporates the intermediate principal stress parameter $b$), local damage failure of the pipeline can be determined. The results indicate that the theoretical predictions are in best agreement with experimental data when $b=0.2$, with a relative error of less than 10%. This approach accurately describes the local plastic deformation and damage behavior of the pipeline, providing a theoretical basis and engineering reference for the safety assessment and protection design of long-distance natural gas pipelines under high-velocity impact loading.