The invention relates to an in-situ protection three-in-one network common port transmission method and an in-situ protection system. The method comprises: when a certain frame of SV message is completely sent at a certain time interval, detecting the type of data to be sent next, and if the type is a GOOSE message or an MMS message, sending the data before the next time interval, and sending thenext frame of SV message at the next time interval; if the type is an SV message, detecting whether there is a GOOSE message or an MMS message requiring sending in the corresponding sub machine, and if so, sending the GOOSE message or MMS message before the next time interval. When the common port data is sent, the SV message is sent at a fixed interval, and the GOOSE message or the MMS message issent once between the transmission of the SV message twice, which solves the problem of the common port transmission of the above three types of messages. The software in the method is simple in processing, high in reliability, easy to implement, and improves the reliability of the in-situ protection device.