Extrusion stress due to phase change of the NiS, internal stress due to CTE mismatch between the NiS and glass, and local concentrated stress around the NiS due to synergistic effects above were quantitatively calculated based on equal strength theory and the deformation coordination relation between NiS and glass in this work. The effects of NiS particle diameter, local strength of glass and ambient temperature on spontaneous breakage of tempered glass were analyzed. The critical diameter of NiS particles was determined as well. The results show that the fundamental reason for spontaneous breakage of tempered glass is the circumferential concentrated stress (tensile stress) near the NiS particle. There exists non-linear relations between circumferential stress and NiS particle diameter of which the circumferential stress decreases rapidly with the decrease of NiS particle diameter when it is less than 0.2 mm, but circumferential stress increases moderately when it is greater than 0.5 mm. The critical diameter of NiS particle which causes the spontaneous breakage of tempered glass decreases with the increase of surface stress in the tempered glass. It is difficult to induce spontaneous breakage of tempered glass when the diameter of NiS particle is less than 0.1 mm. The circumferential stress increases linearly with the increase of ambient temperature, and the stress increases slightly faster for the NiS with larger diameter.