The dialogue between humans and machines is more similar to the oral language with the increasing popularity of intelligent assistants. Therefore, the task of incomplete utterance rewriting (IUR) in multi-turns conversations is a major challenge for machines to understand. To answer user's questions, the machines will extract the omitted information form historical records , reconstruct user's utterances into complete questions and response to the questions. This paper analyzes the performance of different models on incomplete utterance rewriting tasks and compare the differences on generalization ability, data features and data domains. In addition, we propose a knowledge distillation techniques to strengthen the performance of the models, and improve the performance of rewritten questions.