The invention discloses an intelligent on-line oil filling vehicle for a transformer and an oil filling method. The intelligent on-line oil filling vehicle comprises an oil storage tank with an oil outlet valve, a variable frequency oil pump, a three-way pipe and a control module, wherein an oil outlet solenoid valve, an oil drain valve and a pressure sensor with pressure limit control are arranged on the three-way pipe, and the pressure sensor, the three-way pipe, the oil outlet solenoid valve, the oil drain valve, the variable frequency oil pump, the oil storage tank and the control module are arranged on a small car or an electric car; and the oil outlet solenoid valve on the three-way pipe is communicated with an oil filling valve below an oil conservator of the transformer through a connecting pipe in application. The intelligent on-line oil filling vehicle for the transformer can be directly driven to the main transformer of a substation for intelligent live on-line oil filling work, and quickly evacuates from the substation site at the end of work. Compared with the traditional oil filling device, the device has the great advantages of safety, high efficiency, convenience and quickness.