Sensitive material of MQ-2B gas sensor is SnO2, which with lower conductivity in clean air. When the target flammable gas exist, the sensors conductivity gets higher along with the gas concentration rising. Users can convert the change of conductivity to correspond output signal of gas concentration through a simple circuit. MQ-2B gas sensor has high sensitivity to propane and smoke, also can detect the natural gas and other flammable steam well. It is with low cost and suitable for different applications of detecting kinds of flammable gases.