The invention discloses a synchronous phasor measurement method, system and device. The synchronous phasor measurement method comprises the steps of acquiring discrete sampling signals of a power signal; summing the sampling signals corresponding to a target cycle data window, a first cycle data window and a second cycle data window to obtain a target numerical value sum, a first numerical value sum and a second numerical value sum, and then calculating a time constant, a first initial amplitude and a second initial amplitude of an attenuation direct current component; judging whether the target numerical value sum is greater than the first numerical value sum, and judging whether the first initial amplitude is equal to the second initial amplitude; if yes, judging that the sampling signals contain the attenuation direct current component; if not, judging that the sampling signals do not contain the attenuation direct current component; and obtaining a corresponding correction phasor according to the sampling signals, and then determining each parameter of the power signal. According to the method, whether the attenuation direct current component exists or not is accurately judged,the time constant and the initial amplitude of the attenuation direct current component are obtained, the follow-up calculation amount is small, and various parameters of a power signal can be accurately and rapidly obtained.