Abstract: Intelligent combustible gas alarm system is designed based on AVR series microcontroller and catalytic type gas sensor of NAP-100AC. The system is composed of combustible gas controller and detector，NAP-100AC is employed to convert chemical signal into electric signal and this signal will be amplified by circuit，and the detector calculated out the gas concentration of current test area by sampling the output of amplified circuit. Controller communicated with detector by RS-485 bus. The calibration of the detector and the concentration calculation method are introduced in detail，and the reliability of the algorithm is testified by data. The system can monitor the combustible gas concentration within 0%LEL -100%LEL range，and it has the function of dynamic display，alarm and history data query. The system has broad application prospects in the field of urban combustible gas monitoring and industrial combustible gas leaking detection.

Key words: NAP-100AC, catalytic burning, lower explosive limit（LEL）, combustible gas, alarm