In the English language classroom, authenticity is a basic discoursal feature of teachers' instructional discourse.Without authenticity, there will be a mismatch between what students learn in the classroom and what they use in real life. However, the notion of authenticity varies with the change between the natural mode and pedagogic mode in the classroom. Therefore, based on this assumption, retrieving and analyzing data from the corpus and classroom videos, the present paper identifies two categories of problems in Chinese teachers' instructional discourse. Sample extracts are discussed and predictions are made on how teachers' failure to produce authentic discourse may result in ineffective teaching and learning.