The invention discloses a method and a system for reducing the peak-to-average power ratio of an OFDM system based on four-dimensional signal insertion. Based on a conventional DSI method, four-dimensional signals instead of a conventional virtual sequence not carrying information are inserted into OFDM signal sub-carriers, each four-dimensional signal occupies two sub-carriers, and compared withthe conventional DSI method, the data transmission efficiency is not reduced. Since a four-dimensional signal constellation can provide greater freedom of search, the peak-to-average power ratio of the OFDM system can be improved more effectively under the same search complexity and system data transmission efficiency, and no sideband information needs to be transmitted. In addition, compared withthe insertion of two-dimensional signals, the four-dimensional signal constellation has a larger minimum Euclidean distance without changing the average power of the transmitted OFDM signals, so theerror performance of the system can also be improved to a certain extent.