Abstract: An on-line mass spectrometry analysis method was described, which was used in production process monitoring and process control of the coal chemical industry. Compared with traditional chromatography in the coal chemical process monitoring applications, on-line mass spectrometry monitoring system has real-time online measurement component more, fast response, good stability, wide dynamic range. A quantitative analysis method by FPI Mars-550 process gas mass spectrometry was introduced, including the principle, data collection and processing. Taking the real-time monitoring of gas mixture in the synthetic ammonia process as an example, the paper illustrates how to choose the ions for quantitative analysis, and how to do the data processing and analyze the result. The result shows that the relative error by this method is less than 1%, and this is a real-time, fast, accurate method for multistream and multi-component analysis.