Provided are a method and a system for measuring total phosphorus that may predict total phosphorus of a river valley using multi-parameter water quality that are measured in real time through a multi-parameter water quality measuring unit and the like, and may increase the accuracy thereof. The total phosphorus measuring method according to the present disclosure includes: computing a correlation between the multi-parameter water quality and the total phosphorus using multi-parameter water quality data and total phosphorus data measured for a predetermined period; selecting upper parameters having a high correlation from among the multi-parameter water quality based on the computation result; generating a total phosphorus prediction model through a regression analysis between the upper parameters and the total phosphorus; measuring the multi-parameter water quality; and predicting the total phosphorus by replacing the total phosphorus prediction model with the measured multi-parameter water quality.