Cutting transport is a big problem in a wellbore in deep-water drilling specially with a long and large diameter riser. And proper cutting concentration is the key parameter of controlling wellbore pressure and ECD. In this study, a cutting concentration model of a vertical wellbore annulus in deep-water drilling operation has been developed. The model is solved numerically to predict the proportion respectively in each interval of the deep-water well and the rule of proportion changed with time. It is applied in the calculation of cutting concentration in the oil field and a good effect has been achieved.