Objective To describe the inference process of identifying epidemic qi as a disease cause.Methods Clauses related to epidemic qi in Wenyi Lun（ Treatise on Warm Pestilence） written by Wu Youke were rearranged to analyze how he inferred epidemic qi as a disease cause with abductive reasoning. Results The discovery of epidemic qi as a disease cause is a complicated abductive reasoning process based on analogy

which contains Peirce’s adductive model and inference model for the optimal explanation. Conclusion There are some differences between the abduction structure of TCM etiology and western medicine etiology; The etiological hypothesis

which is obtained through the process of abduction

needs to be verified from the perspective of logic.