This paper discusses the differences of the polysemy modality verb “yao/hui” from four aspects on expression of epistemic modality. The bi-ggest difference between the two is that “yao” is based on the perceived symptoms of direct experience, while “hui” can express speculation based on direct experience, but most of them are based on logical reasoning, and their focus is on reasoning. First of all, “yao” is usually co-occurring with a highly dynamic pr-edicate component in a declarative sentence, and “hui” can coexist with a dynamic predicate as well as a static predicate. The subject that co-occurs with “yao” must be definite subject, and “hui” is not subject to this restriction. Second, from the perspective of “evidentiality”, “yao” is based on the perception of impending events, making it easier for the speaker to judge the individual events with changes around it; “hui” is based on some logic. The above speculation, even if there is no perception of a situation that is about to change in front of the eye, can also express the speaker‘s ju-dgment on the state of affairs still in the imaginary world. Third, “yao” often co-occurs with the aspect particle “le” because it expresses a possible state of affairs with strong characteristics of change and urgency. “hui” cannot co-exist with “le” because it has a strong “unrealistic” semantic feature that expresses the epistemic modality. When “yao”expresses epistemic modality alone, its meaning is often uncertain. Therefore, it is often necessary to have the aid of “classification” function “de”at the psychological level of the speaker, and “hui” even if there is no “de”, it will not affect the meaning of its expression of epistemic modality. Finally, based on the corpus survey, we find that the ratio of “yao” to express negative events is much higher than that of “hui.” This paper considers that this is the phenomenon caused by the urgency of time in “yao”.