Abstract: The shear connectors of a composite frame beam will deform in its use phase, resulting in its increased deflection. The traditional methods of a complete interaction to calculate the deflection of a composite frame beam will be unsafe. This paper firstly presents a study on a theoretic model of a composite frame beam considering its slip effect. The basic equations of a traditional beam model TBM and a slip-beam model SLM had been derived according to elastic theory. Then the theoretic models were verified by finite element methods which show the accuracy and reasonableness of the theoretic model. The parametric analysis of theoretical models has been carried out to study the impact of 7 dimensionless parameters on the stiffness reduction factor of a composite beam considering its slip effect. These dimensionless parameters include width-span ratio, width-thickness ratio, height ratio of a concrete slab, etc. It is found that the key parameter influencing the stiffness reduction factor is the degree of shear connection. Then simplified equations of rigidity reduction factors have been proposed for a steel-concrete composite frame beam. The results of the simplified formula fit well with the test results, which will provide useful references to the study and design of composite beams.