Staple yarn is the basic element of most textiles, and its strength affects the performance and processing efficiency of the final textile products. Meanwhile, its strength is the combination of the resistance to breakage and slip of fibers in the yarn. In this paper, a computational model is proposed to predict the strength of staple yarn. The resistance to breakage and slip of each fiber were first computed based on the random arrangement of the fibers and the idealized yarn structure, and the strength was computed by identifying fiber breakage or slip in the yarn breakage zone. The reliability and applicability of the model were verified on viscose and polyester ring-spun staple yarns. The results show that the mean error of prediction is mostly less than 5%, indicating that the model has good accuracy and applicability; and the correlation coefficient R is mostly greater than 0.95, indicating that the predicted strength is in good agreement with the tested strength.