The present invention discloses an online monitoring system, comprising a self-priming pump, a first sample pipe and a container containing a sample to be tested, and the first sample pipe connects the self-priming pump and the container, and the self-priming pump transports the ample to be tested in the container to the first sample pipe; the online monitoring system further comprises a fluid supplementation device connected with the first sample pipe, and a default level value is preset in the fluid supplementation device, and the fluid supplementation device monitors a level of the sample to be tested in the first sample pipe, and extracts the sample to be tested from the container to supplement to the first sample pipe as the level value of the sample to be tested in the first sample pipe is lower than the default level value.