The invention relates to an on-line monitoring system of a milking pulsator for cows. The on-line monitoring system comprises a vacuum pressure sensor, wherein the vacuum pressure sensor is connected with a pulsation pipe of the pulsator and used for detecting change of vacuum pressure in the pulsation pipe; the vacuum pressure sensor is also connected with a single-chip microcomputer control circuit, and the single-chip microcomputer control circuit is used for controlling the acquisition frequency of the vacuum pressure sensor and uploading data acquired by the vacuum pressure sensor to an upper computer; the upper computer draws a pulsator running curve according to the received pulsation frequency, pulsation rate and vacuum fluctuation and judges whether the pulsator is normal or not according to the drawn curve. The system can display all milking positions and the running state of the milking pulsator in real time and can give alarms timely for the abnormal pulsator or the air leaking position of a milking pipeline.