Water quality on-line monitoring system and on-line monitoring method

The invention discloses a water quality on-line monitoring system and an on-line monitoring method, and relates to the technical field of water quality monitoring, the water quality on-line monitoring system comprises a monitoring cylinder body arranged in a discharge pipe, and the right side end of the monitoring cylinder body is provided with a sampling port. Waste water flowing leftwards in the discharge pipe is received through the sampling opening, the waste water is blocked and turned by means of the conical section of the flow guide body, and primary speed reduction is carried out; then, after the wastewater flows into the upper cavity and the lower cavity, the wastewater collides with the left side wall surfaces of the upper cavity and the lower cavity, and secondary speed reduction is carried out; the wastewater in the monitoring cavity can be discharged from the water outlet in the left side of the monitoring cavity, and the flowability of the wastewater in the monitoring cavity can be slowed down d