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The incipient velocity of sediment is one of themost important basic theory problems of hydraulic engineers.The initial motion of sediment is a random process. Based on the combination methods of classical mechanics with statisticstheory the formula to calculate the incipient motion of sedi-ment was established. According to the standard of incipientmotion, motion status, relative degree of expose for sedimentand equivalent grain was defined in this paper. The coefficientin the formula included the flow fluctuation and relative degreeof exposition. The value of the coefficient was calculated by u-sing some parameters value. The results show that the valueof dimensionless shear stress coefficient is not a constant, butlocating in a range from 0. 022 to 0. 063 to weak and middlemotion status, and varying with the relative degree of sedi-ment expose. The value of dimensionless Shields numbers thatput forward in the text can explain the reason that why the co-efficients in difference formulas have wide scatter. The theo-retical formula has been verified with amount of data collectedfrom both natural rivers and flumes. The results can reflectthe motion characteristic of the sediment.