With the increasing of radio systems, the spectrum resources are becoming more and more stringent. Cognitive Radio is one kind of methods to solve this problem and to utilize the spectrum resources not occupied by users adequately. The key idea of Cognitive Radio is to detect idle spectrum resources and make full use of them, so the realization of cognitive radio system depends on the realization of spectrum sensing. Cognitive Radio is a special extent of software radio, uses a new protocol called radio etiquette protocol and provides a kind of mechanism to plan radio spectrum flexibly. Although Cognitive Radio can be used to manage the finite spectrum resources in a more flexible way, in order to using it into practical communication systems, some key technologies such as spectrum detection and spectrum management are needed to be solved. This paper studies the detection of modulation signals within a given spectrum band, further researches the cyclostationarity model and some improved methods about it.