A quartz tube reactor was designed to combine with a commercial mass spectrometer for on-line detecting the gases evolved during pyrolysis of coal and other samples with high volatile. The reliability and repeatability of this atmosphere pressure-temperature programmed pyrolysis-mass spectrum (AP-TPP-MS) system were tested by model compound and real coal sample. The results show that pyrolysis of model compound can give good response, less overlap or tailing gases evolution curves;and multi-peaks can be observed from the same gas curve of real sample. The reliability and repeatability of this system are perfect and the system can be applied to study the coals decarboxylation by comparing the CO_2 evolution curves.