Influences of biogas fuel components, conditions of unburned mixture as well as conditions of pilot flame on the progress of ignition processes in the spherical combustion chamber of biogas-diesel dual fuel engines are simulated by the gas dynamics software FLUENT. The results show that increasing the amount of fuel in pilot flame, shortening injection duration, increasing the advance ignition angle or increasing the temperature of the mixture before ignition will shorten the ignition time for poor biogas fuel. Conversely, increasing the amount of exhaust gas recirculation or reducing the compression ratio of biogas-diesel dual fuel engines converted from the diesel engine is a measure to ensure that the engine runs on rich biogas without knock.