The invention discloses an online tolerance warning method for a parallel capacitor bank. The method comprises the steps that harmonic monitoring is carried out on the parallel capacitor bank; the measured voltage and the measured current of a harmonic are acquired based on original data acquired through harmonic monitoring; according to the measured voltage and the measured current, a first parameter used for characterizing the ratio of the measured voltage to rated voltage and a second parameter used for characterizing the ratio of the measured current to rated current are acquired; according to the first parameter, the second parameter and the series resistance rate of an acquired detected capacitive circuit, the corresponding relationship between the impedance correlation amount of the parallel capacitor bank and the harmonic voltage background content ratio is acquired; and online tolerance warning of the parallel capacitor bank is carried out according to the corresponding relationship. The invention further discloses an online tolerance warning device for the parallel capacitor bank.