“契约必须严守”是合同法上的一个古老的规则,其意在确保双方当事人依诚信按约定履行各自的合同义务。但是,缔约后出现当事人无法预见或避免的客观情况,导致契约无法正常履行,就需要情势变更规则加以调整。本文试从德国的交易基础理论入手,通过对情势变更规则适用的法律效果进行阐述,论说情势变更规则在我国合同法中确立的必要性。 “The contract must be strictly observed” is an ancient rule in contract law that aims to ensure that both parties perform their contractual obligations as agreed upon in good faith. However, after the conclusion of the contract, the objective circumstances that can not be foreseen or avoided by the parties have led to the failure of the contract to be implemented normally and the situation change rules need to be adjusted. This article starts with the basic theory of trading in Germany and elaborates the applicable legal effects of the situation change rules. It discusses the necessity of establishing the rules of changing circumstances in our country’s contract law.