Transformer noise sensor, using DC12V or battery power supply, used to obtain the noise signal of the transformer and convert the sound signal into a voltage signal. The noise sensor compares the voltage signal with the noise threshold voltage, and an early warning signal when the noise threshold voltage value is exceeded The generating device starts and generates an early warning signal, and the generated early warning signal is processed by the intelligent environment comprehensive monitoring device and sent to the online monitoring background. The transformer noise online monitoring system has the transformer noise monitoring function, and can send the early warning signal to the staff's mobile phone in time to deal with the excessive noise of the transformer in time, or repair the transformer when the excessive noise is caused by the transformer fault.