In the mining process of natural gas from a hydrate reservoir, sanding is one of the major factors restricting the development of natural gas hydrate. In this paper, based on the assumption that hydrate cemented sediment is composed of tightly squeezed balls in equal size, we analyzed the stress conditions of such sand particles on the leading edge and in the decomposed zone. A critical velocity model for sand migration in decomposed hydrate-cemented sediment is built then on account of torque balance conditions. The results indicate that critical velocity of particles on the leading edge and in the decomposed zone increases with the increase in particle diameter, arrangement angle and coefficient of friction, and the properties of cement and the type and concentration of electrolyte in the fluid also influence the critical velocity of sand migration. The model proposed in this paper can provide a theoretical basis for evaluation of hydrate sanding.