To investigate the load transferring mechanism of different corner fitting joints under horizontal static load, horizontal static load tests of full-scale frames with four types of corner fitting joints (shaped of L, T, cross, and grouting L) were designed based on the double-layer single span structure of a proposed new assembled composite light-weight slab structure residential project. The working performance and failure characteristics of the steel frames with different joints were studied, and the results were compared with ABAQUS numerical simulations. According to the results of tests and numerical simulations, the initial stiffness and ultimate bearing capacity of the frames, as well as the strain values at different cross sections were obtained. Results show that the corner fitting joints of the new light-weight slab structure worked well, the steel frames broke in the beam hinges, and the joints were intact, which fulfilled the design requirements of "strong column and weak beam" and "strong joint and weak member". The double columns of T shaped joint had a good cooperative performance, and the columns of L and cross shaped joints were prone to deformation. Small diagonal braces and grouting joints increased the initial stiffness of the steel frames by 38% and 14% respectively. The double beams worked independently, and the multiple columns worked incompletely independently. A calculation method for equivalent bending stiffness of double-beam and multi-column was proposed through strain analysis. Numerical simulations agreed well with the tests results, which verified the correctness of the conclusions.