Vocabulary teaching has long been a weak link in teaching Chinese as a foreign language. One of the consequences is the serious shortage of learners' vocabulary. The vocabulary expansion method known as "snowball" is commonly used in textbooks and in teaching practice. However, its strong subjectivity and randomness has been observed, and the morphological rules crucial for the method are not explicitly expressed. The effectiveness of this method is thus largely limited. Through the three examples of morphemes in different positions, this paper studies how to use word formation rules in vocabulary expansion, and puts forward some suggestions on the construction of this method in terms of its contents and procedures. Our research proves that to achieve systematic vocabulary teaching and to let learners master the network relationship between a large amount of isolated words, we must first study the morphological rules of word formation, and then properly integrate these rules into the vocabulary expansion method.