Doubt is undoubtedly significant to knowledge, to the extent that the western epistemology is rooted in philosopher's spirits of doubt(skepticism), which causes the pursuit of certainty to be the force of the development of western philosophy.Knowledge analysis in the contemporary epistemology also complies with such internal mechanism of pursuing certainty, and the exploration of theory of justification would be put in the same skeptical framework.Based on the discussion of skepticism tradition, this paper investigates the significance of skepticism to knowledge analysis and the possible cognitive error, and would try to eliminate the threat of skepticism from the perspective of reliability.