The invention belongs to the technical field of insulating oil for power transformers with high overload capacity, and particularly relates to high-overload transformer oil and a preparation process thereof. The high-overload transformer oil is prepared from natural ester insulating oil and mineral insulating oil according to the volume percentage of (40%-50%):(60%-43%). The preparation process comprises the following steps: firstly, heating the natural ester insulating oil to 80 to 95 DEG C under a vacuum stirring condition; then under a vacuum condition, adding the mineral insulating oil into the natural ester insulating oil, and carrying out mechanical stirring and then ultrasonic oscillation; finally, carrying out two-stage vacuum filtering treatment on treated transformer oil, wherein the closed-cup flash point of a treated oil product is greater than or equal to 155 DEG C. The transformer oil disclosed by the invention is high in physical, chemical and electrical properties, low in kinematic viscosity, high in flowability and long in service life; the transformer with high overload capacity is high in overload capacity; faults of the transformer are reduced, and the problem of 'load returning' is effectively solved; the safety stability of the transformer is improved; the high-overload transformer oil has a high popularization and application value.