Abstract: In order to study the combustion characteristics of biogas engine and the method to control its emissions at idle speed, the low heat thermal cracking biogas was used as the fuel for the biogas engine. Heat release rate, combustion parameters and emission characteristics of the biogas engine at idle speed were analyzed by gathering the indicator diagram and emission index of the spark-ignition biogas engine. The experimental results showed that incomplete combustion and variety of combustion parameters were mainly caused by fluctuation of the charge in the cylinder and difference of the flame expansion. When the idle misfire or incomplete combustion happened, the exhaust of CO at idle speed could reach 4.07%–4.32%, the exhaust of HC could reach 350×10-6–400×10-6 and the carbon exhaust could reach 0.2–0.4 BSU. The abnormal combustion and the emissions at idle speed can be improved effectively by reducing the fluctuation of the charge in the cylinder.