Anew technology of metal surface modification-QPQ (Quench-Polish-Quench) Salt bath compound heat treatment technology can improve theabradability and anticorrosion of metal surface. The CNO-ion plays a very important role in the quality control of QPQ technology. A sequential injection spectrophotometric method is proposed for the determination of CNO-ion. In addition, the indirect detection method of converting CNO-ion into ammonium ion (NH+4) in the condition of high temperature and high pressure is studied; a 697 nm line is selected as the characteristic wavelength. The measuring range of this experiment setup is 0. 02~0. 6 mg·kg-1, and the limit of detection (LOD) is 0.018 mg·kg-1. Based on the sequential injection, the setup can completely automatic on-situ test the concentration of CNO-ion in the quench-polish-quench (techniques for heat treatment of salt bath) process. Measure mean relative error and relative standard variance is 1.31%, 0.92% respectively. Compared with the traditional standard measurement method, a high precision and good repeatability result meets the requirements, which provides theoretical and technical support for the development of the fully automatic in-situ measurement instrument for the treatment of the salt bath compound heat treatment technology.