Early-warning of potential sub-synchronous oscillations is essential to ensure the security of generation units and the stability of corresponding power grid. Based on the principle of phasor measurement unit（PMU）， the impact of sub-synchronous oscillation on synchronous phasor measurement can be expressed as amplitude and phase angle modulation of synchronous phasor. Online monitoring and early-warning method for the sub-synchronous oscillation based on PMU measurements is given as well. Simulation and practical examples based on wide area measurement system（WAMS） at North China Power Grid show that the method proposed is feasible and reliable.