TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: The coordinate region of oxygen-containing coal bed gas is constructed by setting the intersection O(0,0) between horizontal axis and longitudinal axis as origin of coordinate, in which the point with methane volume concentration of 100% and oxygen volume concentration of 0 is set as point B, and the point with oxygen volume concentration of 100% and methane volume concentration of 0 is set as point A, and connecting the points A, O and B such that to form the coordinate region of oxygen-containing coal bed gas during separation process. The corresponding explosion triangle of oxygen-containing coal bed gas under initial state is constructed by setting the intersection between linear equation (Y=-4.785X of oxygen+100) of mixed gas containing methane and air while rectifying and longitudinal axis of coordinate as A1, connecting the points A1 and B to form a linear equation A1B of mixed gas containing methane and air, where X of oxygen is oxygen volume concentration and Y is methane volume concentration, connecting lower limit explosion point L of mixed gas, upper limit explosion point V and critical explosion concentration point N under the condition of normal temperature and normal pressure such that to form explosion triangle LVN of oxygen-containing coal bed gas under normal temperature and normal pressure, where LN is lower limit explosion line of oxygen-containing coal bed gas under normal temperature and normal pressure, and VN represents upper limit explosion line of oxygen-containing coal bed gas under normal temperature and normal pressure, drawing parallel lines of LN and VN according to lower limit explosion point L1 and upper limit explosion point V1 of oxygen-containing coal bed gas under initial state and abnormal temperature and pressure, where the two parallel lines are crossed at the point N0, thereby explosion triangle L1V1N0 of oxygen-containing coal bed gas under initial state is formed, L1N0 is lower limit explosion line of oxygen-containing coal bed gas under initial state, and V1N0 is upper limit explosion line of oxygen-containing coal bed gas under initial state, prolonging N0L1 such that to cross AB at L0, prolonging N0V1 to cross AB at V0, and connecting L0, V0 and N0 to obtain explosion triangle L0V0N0 of oxygen-containing coal bed gas within the coordinate region. The critical explosion point within the coordinate region corresponded to most explosive point is obtained by setting the intersection between linear equation (Y=-4.785X of oxygen +100) of mixed gas and longitudinal axis, connecting lower limit explosion point L of mixed gas, upper limit explosion point V and critical explosion concentration point N under the condition of normal temperature and normal pressure, drawing parallel lines of LN and VN according to lower limit explosion point L2 and upper limit explosion point V2 of mixed gas at temperature and pressure of point corresponding to most explosive point. The two parallel lines are crossed at critical explosion point (N'0) within coordinate region corresponded to most explosive point. Preferred System: The raw gas of oxygen-containing coal bed gas is cooled by introducing into a cooling system, rectifying the oxygen-containing coal bed gas by introducing into a rectifying tower, discharging tail gas from top portion of rectifying system, and discharging pure methane from bottom portion of rectifying system. The most explosive point during separation is close to the outlet of rectifying tower. The tail gas discharged from the top portion of rectifying portion is entered into refrigeration recovering system, and discharged through outlet of recovering system. The cooling system and refrigeration recovering system are provided in a same device. The refrigeration recovering system is throttling device or expanding device.