The invention provides an automatic cut-in device for parallel energy-saving operation of a 10-20KV distribution transformer, comprising a current signal detecting and judging processing circuit, a delay circuit, a circuit for switching on when a transformer is put into use and a circuit for switching off when the transformer is out of operation; the current signal detecting and judging processing circuit outputs a large fixed-value end and a small fixed-value end which are respectively connected with the circuit for switching on when a transformer is put into use and the circuit for switching off when the transformer is out of operation through the delay circuit; when a current detecting circuit detects that the current value exceeds the set large fixed value, the circuit for switching-on is delayed to be started up, and a transformer is automatically put into operation; when a small fixed value is detected, the circuit for switching off is delayed to be started up, and the transformer is automatically out of operation. The device can accurately and automatically finish cut-in in time by a set energy-saving operation way of the transformer according to the electrical load, lower the nonload losses of the transformer, be used for the parallel operation of transformers and the operation of a primary transformer and a secondary transformer, eliminate the complex heavy switching operation and misoperation of working staff, effectively reduce the operation loss of a power grid and improve the benefits of enterprises and the society.