In order to specify the behaviors of structure-complex Petri nets, the concept of synchronous composition is extended and a method is presented, with which a given structure-complex Petri net can be obtained through the synchronous composition of a set of structure-simple Petri nets, namely S-nets. Firstly, the language characters of S-nets are analyzed with details and the methods to obtain their language expressions are presented. With the synchronous intersection operation of Petri net languages, the language relationships between the structure-complex Petri net and the set of S-nets can be expressed. Based on these works, an algorithm to specify the behaviors of Petri nets especially structure-complex systems is obtained.