The theoretical model to compute long-term stress of steel-concrete composite beam was deduced. On this basis, the effects of factors such as concrete age to loading, longitudinal reinforcement ratio in concrete slab, concrete slab width, environmental yearly average relative humidity and concrete strength on long-term stress of composite beams are discussed. The results show that additional stress at the top of concrete slab is tensile stress and that at the bottom of steel beam is compressive stress. Concrete slab width, as well as longitudinal reinforcement ratio can not be ignored. Concrete strength and concrete age to loading have relatively bigger effect as well. But environmental yearly average relative humidity has less impact.