The utility model provides a measuring device for water quality chemical oxygen demand with ozone method for collecting total light quantity. The main body of the measuring device is a light-proof reaction cavity body; the reaction cavity body is provided with a water inlet, a water outlet and a through hole; one side of the reaction cavity body is provided with a lens; an aeration piece is arranged below the reaction cavity body; the aeration piece is communicated with an ozone air chamber; the water inlet and the water outlet are connected with a water sample switching device; a photoelectric sensor is arranged at the focus of the lens outside the reaction cavity body; the outlet signal port of the photoelectric sensor is connected with a data processing circuit; the data processing circuit is connected with a control circuit; and the control signal output end of the control circuit is connected with the water sample switching device. Chemical aerobic reaction is carried out on the water sample with a fixed volume by continuously applying ozone, and the water quality chemical oxygen demand is accurately measured by measuring the total light intensity generated in the reaction process. The measuring device is applied to the measurement of water sample with high COD (Chemical Oxygen Demand) value; and the measurement result does not depend on the flow rate of the pump and has high test precision.