The invention relates to an online water quality analyzer. The online water quality analyzer includes a meter box, a mounting plate, a reagent pump, a sampling pump, a mixing pump and a colorimetric measurement unit, wherein the colorimetric measurement unit includes a cuvette, a light source and a light receiver, the sampling pump is in communication with the mixing pump through a sampling pipeline, the reagent pump is in communication with the cuvette through a reagent pipeline, the cuvette is in communication with the sampling pipeline through a first mixing pipeline, the mixing pump is incommunication with the cuvette through a second mixing pipeline, through setting the mixing pump, the test reagent extracted from the cuvette by the reagent pump and a to-be-sampled sample extracted by the sampling pump are thoroughly mixed and then recycled to the cuvette for detection, manual mixing is not needed, high degree of automation is achieved, in addition, the sampling pump, the reagentpump and the colorimetric measuring unit are integrated into the mounting plate in the meter box, sampling, mixing and detection are integrated into one, high integration and small size are achieved,and the online water quality analyzer is easy for operators to install and use.