The invention relates to a method for preparing a natural gas by using a coke oven gas as a raw material and using a biological method, and in particular provides a method of removing CO, CO2 and H2 in the coke oven gas or increasing the removing rates of the CO, the CO2 and the H2 in the coke oven gas. The method comprises the following steps: the CH4-containing coke oven gas is subjected to anaerobic fermentation, so that the CO, the CO2 and the H2 in the coke oven gas are removed, wherein after the coke oven gas is purified, the oxygen content is equal to 10ppm or less, and CO2 is supplemented; and preferably, the fermentation is continuous fermentation. The invention also provides the method for purifying CH4 or preparing the natural gas by using the coke oven gas as the raw material and using the biological method, and a methane-enriched gas and the natural gas obtained by adopting the above method.