Platform enterprises have made considerable revolutions in network environments in recent years．Studing its operation strategy from a proper perspective is an important scientific issue． Selecting the practical scale and expected scale of platform ecosystem and time as three parameters，the paper constructs the curve of life cycle of platform ecosystems． The curve is segmented into several stages through some nodes，and the scale，network effects and development patterns of platform ecosystem embody different characteristics in every stage． These characteristics offer the references to formulate operation strategies for platform enterprises． Generally speaking，when platform enterprises could not take full advantage of network effects，potential participants should be attracted to enter into platform ecosystems through“non-network effects inducements”; conversely，platform enterprises should make use of the network effects and develop at a full speed，and should also deal with the problems emerged in the development． This study sets up a theory framework of operation strategy for platform enterprises and develops life cycle theories of platform ecosystems． The conclusions also have guiding significance in making self-diagnostic and operation strategy for platform enterprises．