Abstract: The study of petroleum system forms one the hot subjects in petroleum geology,and plays an important role in oil/gas exploration.The petroleum systems in nonmarine multi-cycle composite basins in China shows much complexity with multiple sources,multiple kitchens,multistage hydrocarbon generation and pooling.Such complex petroleum system has three obvious features:① superimposition and interlacing one after another of the system as a result of multiple sources and multiple kitchens;②mixed oil generation arising from multistage hydrocarbon generation and pooling;③multiple critical time.Repeated large-scale oil/gas migration would usually have occurred in the process of evolution of the complex petroleum system,so it is necessary to take into consideration their restoration.Finally,the paper discusses the characteristics of the complex petroleum system in particular,citing the Mahu-penyijingxi complex petroleum system of junggar basin as an example.