The utility model provides an on-site monitoring and alarm instrument for a transformer. The on-site monitoring and alarm instrument comprises a power supply, a signal sensor chip, a data processor and an alarm, wherein the signal sensor chip and the data processor are connected between the transformer and the alarm through connecting leads, the data processor is connected with the alarm, and the connecting leads are led out from the data processor. When a client consumes power illegally, the on-site monitoring and alarm instrument gives off alarm and promotion to prevent illegal power consumption at will. The on-site monitoring and alarm instrument is convenient for management of transformer, and timely contact power management department after abnormal situation occurs. The inner and outer structures of the transformer are monitored to facilitate power inspection by an inspector. Illegal power consumption can be observed directly by an on-site inspector without approaching to the transformer, thereby improving inspection safety.