In this paper, based on analyzing some existing phase lock detectors for quadratic phase-shift keying (QPSK),we propose a novel method of estimating the signal-to-noise ratio (SNR) operating with the lock metric value of classical Mth-power (M=4) phase lock detector. The proposed lock detector-based SNR estimator can perform better than the conventional SNR estimator in terms of mean estimated value (MEV) at medium to high SNR when phase recovery loop is in-lock status. And its normalized mean squared error (NMSE) can reach Cramer-Rao bounds (CRBs) at that SNR. Moreover, the computational complexity of new estimator operating in phase recovery loop is analyzed with less additional multiplications than that of conventional one. This estimation method can also be applied to other similar phase lock detectors.