The invention discloses an internal communication drainage method and system based on an SDN (Software Defined Network) for a transformer substation. The method comprises the following steps: determining a first switch to be passed by business data needing to be drained; controlling the first switch to mirror the business data needing to be drained; controlling the first switch to transmit the mirrored business data to a second switch; and controlling the second switch to send the mirrored business data to an analysis terminal. According to the communication drainage method and system providedby the invention, a mirror port is partitioned on a business switch (that is, the first switch) and connected to an aggregation switch (that is, the second switch), and a mirroring operation of datastreams and a specified operation of a drainage path are carried out through centralized control of a controller, so that a fine-granularity business service for internal communication of the transformer substation is realized, and the automation degree and intelligence degree of the transformer substation are raised.