Raw milk is susceptible to bacterial contamination. While low-temperature storage inhibits the growth of most bacteria, psychrophilic bacteria can compromise dairy quality. Their complex community structure and diverse sources pose significant challenges for control. A total of 120 samples from 24 farms in Shandong, including raw milk, udder swabs, feed, tank water, and groundwater, were analyzed. All samples were collected from environments directly associated with the raw milk production process, where bacterial control measures could be readily implemented. SMRT sequencing and SourceTracker analysis identified groundwater as the main source of bacteria in raw milk. Groundwater can introduce bacteria into raw milk through the washing of cow surfaces and the cleaning of refrigerated tanks. High-abundance psychrophilic bacteria in raw milk included Pseudomonas, Lactobacillus, Acinetobacter, and Streptococcus pneumoniae. SouceTracker analyses found that the main source of Pseudomonas was not in an environment where bacterial control measures could be easily implemented. This finding analyses the reasons for the prevalence of Pseudomonas contamination in terms of the traceability of psychrophilic bacteria. Pseudomonas can be removed from raw milk (not source control) by methods such as CO