As memory device packaging moves toward die stacking technology with thin chips of high density, wafer dicing has recently become a key factor in semiconductor yield. Stealth dicing technology, which produces laser irradiation inside the wafer to form a modified layer within the wafer without any damage to the surface, is an alternative and advanced dicing process that is in line with the future trend of thin wafers. The existing stealth dicing process that utilizes an ultrashort pulse laser is expensive. In this study, we proposed a low-cost method for wafer stealth dicing using quasi-continuous wave (QCW) lasers. We redesigned the focusing lens of a QCW metal-cutting laser machine to form a cutting head with a high numerical aperture (NA) that is suitable for wafer stealth dicing. An optothermal model used to simulate the dicing process and calculate the temperature was implemented numerically based on the Crank–Nicolson algorithm and the successive over-relaxation method. Both the simulation and experimental results demonstrate that a QCW laser machine with a high-NA focus lens is ideal for wafer stealth dicing.