Abstract: The effectiveness of knowledge representation methods and knowledge reasoning methods are the basis for the correct decision-making of expert systems for crop pests and diseases. The advantages and disadvantages of traditional representation methods，modern representation methods and hybrid representation methods for common knowledge of expert systems for crop pests and diseases were summarized，and its applications were concluded. Furthermore，the trend of development was analyzed and prospected. The hybrid representation methods applies widely，since they can overcome the limitations of single representation and make full use of the advantages of various methods effectively. The neural network knowledge representation indicates good application prospects due to its fast，high accuracy and high flexibility.

Key words: crops, pests and diseases, knowledge representation, expert system