Abstract: Pilot Symbol Assisted Modulation (PSAM) is widely used in the digital burst transmission system. The frequency estimation of a Pre+Post pilot structure departs from its CramrRao Lower Bound (CRLB) when frequency error is large and SNR is low. In this paper, the distribution of the main lobe and side lobe of the maximum likelihood measurements is derived for the estimation of symmetric pilot structures frequency and the estimation ambiguity issue is analyzed. Then, a novel mixed pilot structure is proposed and the principles of selecting parameters are given. This new pilot structure can deal with high Doppler frequency shift with high precision under low SNR. The simulation results show the advantages of this pilot structure.