In order to derive the load slip constitutive expression of headed stud connectors used widely in composite bridges, thirty five push out specimens were prepared and tested. Based on the tests results, the influences of stud diameter, stud height, stud ultimate tensile strength and compressive strength of concrete on the peak slip were investigated, and the expression for calculating the peak slip and the load slip constitutive expression with a consideration of the effect of peak slip were proposed. The results indicate that the peak slip increases with the increasing of stud diameter, but stud height and ultimate tensile strength decreases with the increasing of compressive strength of concrete. The proposed expression of peak slip considers more influence factors and fits better with the test results in comparison with the expression specified by the CEB FIP. The load slip constitutive expression by taking into consideration the effect of peak slip is able to reflect the influence of stud diameter, stud height, stud ultimate tensile strength and compressive strength and be employed to conduct the non linear numerical and analytical study of composite bridges.