Abstract: At present, the deflection of composite steel-concrete beams under long-term loads is mainly analyzed by the reduced elastic modulus in the national code. The shrinkage and creep of concrete are not considered adequately, also, the interface slip between steel and concrete should not be ignored for long-term deflection. Therefore, a non-linear deflection formula of composite steel-concrete beams under long-term loads is deduced by the main influent factors such as shrinkage, creep, and interface slip, considered and synthesized by previous researches. Two representative composite beams of a tall building are also monitored in situ to study the non-linear deflection, the deflection values of the theoretical calculation and measurement show that the values calculated by the code are smaller than that of the theoretical calculation and the practice. Thus some suggestions for the design and construction of composite beams are proposed.