Hypotheses introduction is the most important characteristic of the system of formalized natural inference.In the natural deduction,the hypothetical premise can be introduced at any time when needed,but it must be eliminated finally because the conclusion can not depend on it.It is a process of syntactic transformation to use the rule for hypotheses elimination to do an inference according to the rule＂If A1,A2,…,An├ B,then A1,A2,…,An-1 ├ An→B＂,but to understand the principle of work of the rule for hypotheses elimination relates to the discussion on the truth-value relation between premise and conclusion,which belongs to the range of semantic interpretation.