Compared with traditional mineral insulating oil, natural ester insulating liquid has become an ideal alternative medium for transformer insulation due to its high fire point and biodegradability. The retrofilling of transformers with natural esters has been gradually promoted and applied. However, there are few studies on the influence of natural ester retrofilling on the insulation strength and heat dissipation performance of distribution transformers. Moreover, the current process flow of factory-returned retrofilling tests requires a lot of time and manpower, which is difficult to meet the requirements of time-limited power outage operations for distribution transformers. This study carries out retrofilling tests on three 630 kVA/10 kV distribution transformers with natural ester using an integrated device. Electric field and thermal simulations are conducted before retrofilling to evaluate the risk of natural ester filled in serviced mineral oil distribution transformers. After retrofilling, the properties of insulating liquid were tested and compared with that before retrofilling. The routine tests and heat run tests were carried out on the transformers. The results show that the insulation resistance of the three transformers decreased after retrofilling, but still above 300 MΩ meeting the requirement limits. The fire point were all greater than 300 °C after retrofilling, meeting the K-class flame retardant requirements, and the residual mineral oil content were estimated from 3.07 wt% to 5.17 wt%. These three retrofilled transformers were all connected to the grid.