The electrical transformer online monitoring system is made up with single chip system, data collection circuit, control circuit, display circuit, alarm circuit, communication circuit and so on. By using numerical temperature sensor, simulation input channel of cooling fan and signal changing circuit of oil velocity, oil temperature, working current of cooling fan and oil velocity are sent to the single chip. The system will compare the data with the enactment value. If it is abnormal, alarm signal will be sent out and transfer to the main control room or dispatching center by serial communication. This system invites a clock circuit for recording the real-time working parameters of transformer accurately in order to check or maintain conveniently. This system can greatly improve the ability of real-time monitoring the transformer and the reliability of power system.