<p>The delineation of groundwater drinking source protection areas is an important link to ensure the safety of drinking water. However, the current methods for delineating protection areas generally assume that the aquifer is homogeneous or zoned according to homogeneity, resulting in the delineated protection areas that may still present a certain risk of pollution. In fact, the aquifer exhibits significant heterogeneity and its spatial distribution is fraught with uncertainty, making it difficult for groundwater protection professionals to identify preferred flow paths that may facilitate the rapid migration of contaminants. Determining spatial heterogeneity within aquifers through field measurements requires significant technical and economic inputs and is therefore not feasible in practice. In this paper, a new method based on Monte Carlo method is proposed to delineate the secondary protection area of groundwater water source. Under certain statistical characteristics, 100 random parameter fields are generated by Monte Carlo method. On this basis, the groundwater numerical simulation is constructed to identify the recharge range of the extraction well, and the superposition of results under heterogeneous fields is determined as the final secondary protection area. This study takes a valley plain water source as a case to conduct a preliminary verification of the method. Through the analysis of the simulation results under 100 random parameter fields, the area of the final secondary protection area is 1.66 times larger than that under homogeneous conditions. This study provides a more reasonable theoretical basis for the delineation of groundwater source protection areas.</p>