This application relates to an encoding method and device, a decoding method and device, and a signal transmission system. The encoding method includes: encoding 8-bit data corresponding to a to-be-encoded byte of to-be-transmitted data into 9-bit data, the to-be-transmitted data comprising at least one to-be-encoded byte; detecting the first digit of data of the 9-bit data and a previous digit of data adjacent to the first digit of data, when the to-be-encoded byte is not the first byte of the to-be-transmitted data; inverting the 9-bit data and then adding a tenth digit of data for indicating that the inverted 9-bit data has undergone an inversion operation behind the inverted 9-bit data to obtain 10-bit data, when the numerical value of the first digit of data is the same as that of the previous digit of data; and adding a tenth digit of data for indicating that the 9-bit data has not undergone an inversion operation behind the 9-bit data to obtain 10-bit data, when the numerical value of the first digit of data is different from that of the previous digit of data.