A novel phasor measurement unit（PMU） with free scaled synchronized time-mark has been developed. The novel PMU is composed of frequency auto-adapted equal interval sampling module， synchronized time-mark generating module， microprocessor core module， and capable of resolving the problem of uncoupling treatment to“time-synchronized” and“frequency-synchronized” sampling. With helps of the synchronized time-mark and its synchronization model set freely， the full circle sampling data can be automatically orientated and intercepted（no frequency spectrum leak out） to make high precision DFT calculation. The phasor calculation offset caused by synchronized time-mark offset can be precisely corrected with simple linear method. The precision of phasor measurement under three time-mark points（the first， middle and last sampling point） of time-mark synchronization has also been simulated， and the best time-mark synchronization point be“the middle sampling point” is verified.