Salt bath compound heat treatment technology is a new kind of metal surface treatment technology, which can enhance the wearability, corrosion and fatigue resistance of workpiece, and it is widely used. In order to guarantee the quality of metal surface treatment, it is very important to accurately determine the content of the Cyanate, Cyanide and Iron ion in nitriding salt. At present, it is difficult for the chemical titration method adopted by industry to satisfy the requirements for the automated analysis. Based on the spectrophotometric method, a semi-automatic nitriding salt parameter detection setup using 510, 620 and 697 nm of three different wavelengths of monochromatic LED light source, a coupling optical fiber and three photodiodes, which can fastly and exactly test the content of the Cyanate, Cyanide and Iron ion. The semi-automatic nitriding salt parameter detection setup includes the Optical system, mixing control system, constant temperature control system and data acquisition circuit. By the indirect method determining the content of Cyanate, converting the Cyanate ions into ammonium ions by chemical method, and then measuring the content of ammonium ions in water according to the national standard GB 7481—1987 Water quality-Determination of ammonium-Spectrophotometric method with salicylic acid, the characteristic absorption wavelength of ammonium is 697 nm. Determining the content of Cyanide according to the national standard HJ 484—2009 Water quality-Determination of Cyanide-Volumetric and Spectrophotometry method, the characteristic absorption wavelength of Cyanide is 620 nm. Determining the content of iron according to the national standard HJ 345—2007 Water quality-determination of Iron-phenanthroline spectrophotometry, the characteristic absorption wavelength of ammonium is 510 nm. In this setup, the light intensity stability of LED light source is tested, and the LED light intensity is a stable value when it starts working. Testing the influence of the coupled optical fibers, the spectrum of the monochromic LED light has not changed between through a coupling fiber and a single fiber, and just the optical intensity decreases a little when the light through the coupling fiber. Testing the stability of the LED light by stirring, the data showed the mixing control system has no effect on the optical system. Using the experimental device to measure the absorbance of Cyanate standard sample-potassium cyanate, Cyanide standard sample-potassium cyanide and iron standard sample-ferrous sulfate, based on the lambert beer’s law, establishing a Cyanate, Cyanide and Iron standard sample fitting curve, whose linear correlation R2 is 0.990 7, 0.999 6, 0.998 1, respectively, which has high linearity. The maximum mean relative error and maximum relative standard deviation of Cyanate, Cyanide and Iron in predicted samples were 4.53% and 1.04%, 2.29% and 0.79%, 4.2% and 0.7%, respectively. The limit of detection of Cyanate, Cyanide and Iron were 0.017, 0.009 and 0.005 mg·L-1, respectively. By contrasting the traditional chemical test method, the experiment setup test the Cyanate, Cyanide and Iron ion in nitriding salt, the test result of the detection system is superior to the traditional chemical titration method, and the maximum mean relative error and maximum relative standard deviation of Cyanate, Cyanide and Iron in nitriding salt sample were 4.17% and 0.69%, 1% and 0.58%, 4% and 0.29%, respectively. The results of these tests meet the design requirements and provide theoretical and technical support for the tri-component semi-automatic analyzer for salt bath compound heat treatment technology. The optical path system of experiment setup includes three monochromatic LEDs light source and a coupling fiber, which realizes the rapid and accurate detection and multi-parameter detection of the light source spectral. The whole optical inspection system has no moving parts, which greatly reduces the system error brought by the optical detection system, which guarantees the accuracy and repeatability of the analyzer test.