The invention relates to a mechanical seal online monitoring system which comprises a data processing unit, a detection unit and a user terminal, wherein the detection unit is connected with the data processing unit; and the detection unit comprises an I / O (Input / Output) module, and a motor rotating speed sensor, a current sensor, a radial vibration sensor and a noise sensor which are connected with the I / O module. In the system, acquired data is subjected to the multivariable statistics process monitoring by the data processing unit, so that the indirect monitoring for the mechanical seal operation condition is realized and an early warning signal can be given in the fault early phase of the mechanical seal, and thus, a production accident is avoided, the loss of raw materials, equipment and other aspects of an enterprise is reduced and the possibility is provided for detection of the mechanical seal state and failure prediction. Meanwhile, the equipment is checked by the monitoring system without shutting down, so that compared with the prior art, a great amount of time and human resources are saved and the production efficiency of the enterprise is improved.