SCHC (SI-CAI hybrid combustion), also known as spark-assisted HCCI, has been proved to be an effective method to stabilize combustion and extend the operation range of high efficiency, low temperature combustion. The combustion is initiated by the spark discharge followed by a propagation of flame front until the auto-ignition of end-gas. Spark ignition and the spark timing can be used to control the combustion event. The goal of this research is to study the effect of flame propagation on the auto-ignition timing in SCHC by means of chemiluminescence imaging and heat release analysis based on an optical engine. With higher EGR (exhaust gas recirculation) rate, more fuel is consumed by the flame propagation and stronger correlation between the flame propagation and auto-ignition is observed. It is found that the auto-ignition event is better correlated to the ignition delay and the normalized flame area, which represents the flame development speed, rather than to the mean heat release rate during the flame propagation stage. While the cumulative heat release rate from the start of combustion to the auto-ignition timing shows no obvious correlation with auto-ignition timing. The combustion phasing and the resulting exhaust gas temperature are able to explain the cyclic evolution of the auto-ignition timing to some extent. However, they are not capable to explain the relationship between auto-ignition and flame propagation.