In order to verify the hardware safety integrity level of a functional safety gas detector used in the explosive atmosphere, a method of FMEDA is adopted to analyze the failure modes and effects and calculate the random hardware failure rates of the gas detector elements, the functional safety related parameters are quantified based on the adopted fault online diagnosis measures. The analysis results show that the safe failure fraction of the gas detector which is 1oo1 architecture, B type system and low demand mode of operation reaches 94%, the average probability of a dangerous failure on demand of the safety function is equal to 6.8×10-4, therefore the hardware design of this functional safety gas detector satisfies the requirements of the safety integrity level SIL 2 specified in the standard IEC 61508.