The invention discloses an online voltage stability early warning method based on Thevenin equivalence, including: S1, establishing a Thevenin equivalent system S2 between each unit generator on an online voltage system side and a target node voltage; S2, according to the Thevenin equivalent system in the S1, calculating the sensitivity of the electromotive force of each unit generator on the online voltage system side in the S1 to the target node voltage; S3, calculating an online node voltage stability margin value based on the sensitivity in the S2; and S4, setting a threshold value, comparing the stability margin value in the S3 with the threshold value, obtaining an online voltage stability result based on a comparison result, and giving an early warning for a circuit network where the online voltage is located according to the result.