The utility model discloses a little water online processing system among transformer oil, including dehydrator, flow indicator, oil pump, little water sensor, filter, exhaust apparatus, fuel tap, electric electrical control unit. Transformer bottom oil drain port passes through filling valve, the little water sensor of oil feed, oil pump, flow indicator and dehydrator one end intercommunication, and the dehydrator body other end communicates through little water sensor that produces oil, filter, exhaust apparatus, delivery valve and transformer tank top, and the dehydrator volume can be according to transformer capacity customization, the inside drier of filling. The utility model discloses can filter off the moisture that dissolves and the little water among the solid insulating material among the transformer oil according to continuous online mistake of predetermined cycle. Can improve the inside operating environment of transformer, effectively improve transformer insulating material's breakdown voltage, it is situated between to reduce decreases, delays aging rate to reach the mesh in extension transformer life -span.