The invention discloses a dual-wavelength ultraviolet method organic wastewater chemical oxygen demand (COD) detection device and a method. In existing actual application, the problem of interference of suspended matter fluctuation in a water sample on absorbance generally exists in a forecast model, so that accuracy of the model is directly affected. A measuring groove is connected with a first monochromator, a second monochromator, an ultraviolet detector and a visible light detector, the first monochromator is connected with an ultraviolet light source, and the second monochromator is sequentially connected with a visible light source, a subtracter, an amplifying circuit, an alternating / direct (A / D) switching circuit, a programmable logic controller (PLC) control system and a display device. The dual-wavelength ultraviolet method organic wastewater COD detection device can accurately forecast COD value in the water sample, is full-automatic in measuring process, greatly shortens measuring period, improves COD measuring speed, improves suitable range and measuring precision, and can be suitable for on-line, rapid, and accurate analysis and measuring of wastewater COD.