Complex networks is a new method to research complicated system from the global perspective. The relation words is the important role in complex sentences. This paper uses complex networks to research the relation words in modern Chinese complex sentences. It finds that complex sentences are linked by the relation words' collocation, so the complex networks which the relations words built is also has the small-world and no-measure characteristics, and it is reflected the relation words' collocation ability and object. This paper discusses the characteristics and rules of the logic semantic relationship in complex sentences through studing the characteristics of the collocation networks of relation words.