The invention discloses an early warning method for voltage sag based on electric energy quality data mining at a monitoring point. The method includes the steps that (1), the weight of all voltage sag amplitudes and duration time is determined through an AHP method; (2), original data are collected, and voltage sag historical monitoring data at the monitoring point are collected; (3), an original data matrix is built, wherein voltage sag test data at the monitoring point, standard voltage, self-setting grade limit values, a voltage sag average value of a previous period of time and the lowest voltage sag limit value are combined together to form the original data matrix; (4), the data are preprocessed, and preprocessing includes same trending and normalizing; (5), all approach coefficients are calculated through an improved TOPSIS method; (6), a early warning grade is judged. The approach degree of the monitoring data and limit values of all grades is calculated by using the improved TOPSIS method, according to the approach degree, the severity degree of voltage sag at the monitoring point is warned early reasonably in time, the voltage sag conditions of a bus or a circuit in a period of time in the area where an electricity supplier and an electricity user belong can be learned, and reliability and economy of power grid operation are improved.