The proper application of the theory of least effort in simultaneous interpretation can ease the burden of interpreters, make audiences obtain the key information in an easier way as well as make the interpretation conform to the language rules of the target language. Through the analysis of the speeches by some leaders and the interpretations, this thesis is trying to prove that concepts such as the deverbalization, cognitive baggage, environment, context as well as cognitive context are beneficial to the proper application of the theory of least effort in simultaneous interpretation. Deverbalization can help interpreters free from the restrictions of the single words in the original speech and obtain the key information, resulting in the proper application of the theory of least effort. Cognitive baggage, environment, context and cognitive context can help interpreters make full use of their knowledge about the world to strengthen their understanding and prediction of the original speeches, which is conducive to the proper application of the theory of least effort.