The quality of optical material cutting affects the efficiency and quality of follow-up polishing processing directly. Among various ways of cutting, the diamond wire saw cutting of fixed abrasive has advantages of narrow cutting seam, efficient cutting, high chip quality, little pollution to environment, ability to process bigger diameter workpiece, etc. So it has been used widely in optical material processing. In this paper, the coupled algorithm of FEM/SPH has been used to simulate deformation, fracture of brittle optical material in diamond grain cutting. The influence of workpiece feeding rate, cutting speed and cutting depth on the surface roughness are then analyzed in detail. Numerical simulation results show that the coupled algorithm efficiently explains the mechanism of material removal. From these results, we can not only determine optimum technology parameters for the manufacture of diamond wire saw, but also provide basis for improving the wire cutting efficiency and finished-product rate of optical material.