The invention discloses a synchronizing method of a high-speed mobile OFDM system. The method comprises the following steps: utilizing the first 10 identical short lead codes in a frame structure of a wireless LAN (local area network) protocol to determine the coarse synchronizing location; according to the coarse synchronizing location, extracting 2 identical long lead codes in a receiving signal and combining with a sliding window to perform fine synchronization; converting the transmitted known long lead codes into a time domain; calculating the mutual correlation value of the extracted long lead codes and the transmitted known long lead codes; determining the maximum of absolute value of the mutual correlation value and the location index corresponding to the maximum; setting the initial value of the preset sliding window; determining the sliding direction of the preset sliding window according to the location index corresponding to the maximum and the set threshold value index, performing fine synchronization, and determining the initial position of the long lead codes. The synchronizing method is applicable to accurately determining the synchronized position of the received information symbol for the OFDM system under the high-speed mobile vehicle-mounted IEEE 802.11p protocol.