Abstract: Synchronization detection plays the most important role in the underwater acoustic communication systems.It is discovered that the STLFM signal has double energy peaks in their 'optimal' fractional Fourier transform domain and these two peaks have symmetry in their amplitude if the STLFM signal is captured in the accurate time windows for the synchronizing. By taking its unique feature into account, a synchronization detection method based on the STLFM(Symmetrical Triangular Linear Frequency Modulation) is proposed in this paper. Both the simulation in multipath channels and outfield experiment results in the Wu-Yuan bay indicated its feasibility and superiority. It is shown that with the same product of pulse width and bandwidth, the proposed method has better accuracy and stability in the synchronization compared with the traditional method, for detail, the error mean has 1/3 reduction and the error variance has reduced to 1/5¹/10.