The invention discloses a river water quality evaluation method based on a phytoplankton-integrity index (P-IBI). The method is used for evaluating and classifying river water quality based on the phytoplankton-integrity index. The invention finds that the P-IBI consisting of total phytoplankton density (N), chlorophyll a (Chl a) and an Odum index can effectively distinguish the difference of mainenvironmental factors in different water quality categories and is significantly positively correlated with a water quality index (WQI) which is originally calculated by measurement of many parameters, so that the P-IBI index consisting of the total phytoplankton density, the chlorophyll a and the Odum index can be adopted to realize rapid evaluation and classification of the river water quality.The method disclosed by the invention is simple and easy to operate, low in cost and easy to popularize and has very obvious advantages.