Flexible decision-aided maximum likelihood (DAML) phase estimation method is proposed, which extends from the phase shifted keying (PSK) system to the quadrature amplitude modulation (QAM) system, and the phase estimation error is derived. Simulation results show that the flexible DAML has a lower bound of bit error ratio (BER) for different block lengths, which can eliminate the block length effect from the conventional DAML and effectively reduce the requirement of signal phase fluctuation in the process of signal detection, thereby it loosens the restriction on laser linewidth in coherent optical systems.