YSI(Multi-parameter water analyzer) is widely used to determine phytoplankton chlorophyll-a concentration in many fields because of its rapidness and portableness. The purpose of this study is to evaluate the efficacy of the YSI Environmental Monitoring System for water quality measurements and data collection by comparing two groups of data using laboratory method and YSI detections respectively. It shows that multiprobe readings are significantly different from laboratory anaiysis of chlorophyll. Mostly, multiprobe readings are much lower than laboratory analysis. All the data are mainly correlated in winter than summer. Linear regression of these two groups of data that were sectionalized into several intervals according to laboratory analysis of chlorophyll indicated that the disparity between these two groups of data becomes bigger with the increase of chlorophyll concentration. Errors are caused possibly by several factors:(1) calibration method of YSI, (2) existence of other kinds of pigment that fluoresces, and (3) the structure of YSI.