In order to realize high-precision synchronization of OFDM (Orthogonal Frequency Division Multiplexing) under arbitrary carrier frequency offset, this study proposes a system timing offset estimation method based on auxiliary data. Firstly, under the Gaussian white noise channel, based on the auxiliary data without special structure, the optimal synchronization algorithm is derived under the maximum likelihood criterion. Then, under the consideration that the maximum likelihood method is too complex, a sub-optimal method, which has reduced computational complexity, is proposed. Finally, the performance of the proposed timing method is evaluated by Monte Carlo simulations under the frequency selective Rayleigh fading channel. The simulation results show that the timing performance of the proposed method is significantly better than the traditional algorithms.