Abstract: According to the practical condition, the calculation model of composite deck slab is built to investigate its fire resistance behavior. Analytic method is adopted to derive the formulas of restraint rigidities of end-supports, section stiffnesses and internal forces. The changing regularity of internal forces of composite slabs is obtained subjected to gravity load, temperature differences and temperature increments. The analysis shows that, compared with the eccentric compression stiffness, the horizontal restraint rigidity of secondary beams to slabs is negligible; before plastic hinges forming, large negative bending moments at supports are caused by temperature differences, resulting in a negative bending moment distribution along the whole span; after plastic hinges forming at supports, the bending moment at the mid-span turns to positive. At last, some suggestions on fire resistance of composite slabs are put forward for researchers and designers.