With further development of restructuring in power industry, it's urgent and necessary to establish multilevel power market in large interconnected power systems in order to better configure energy resources in a larger scope and gain greater economic benefit of power exchange. This paper puts strong emphasis upon system security coordination problem in a multilevel market, which is one of the key problems encountered in such environment. After careful analysis and comparison of several proposed alternatives, a sequential security coordination scheme that is based on two sets of coordination information is recommended. Under this scheme the upper-level and lower-level power market can cooperate smoothly for the system security, and potential power exchanges among lower-level markets can be greatly boosted. The related topics and results of a case study are also introduced. It is concluded that this scheme is excellent for its high economics, low complexity, fair flexibility and easy generalization.