In cast-in-situ Reinforced Concrete (RC) frame structures，the strengthening effect of monolithic slab on the structural integrity makes actual axial force in frame beam cannot be ignored． The axial force distribution of frame beam along beam span in regular RC frame structures subjected to vertical and lateral loading were respec- tively detailedly analyzed． Moreover，the axial force distributions corresponding to different arrangement of mono- lithic slabs in RC frame structures were also studied and compared with each other． The research results show that: monolithic slab is the main reason of beam axial force uneven distribution in actual RC frames and the axial force presents as axial compression force at beam end with hogging moment which cannot be ignored during design stage; the influencing way of monolithic slab is to limit the axial deformation of beam and consequently generate beam axial force; monolithic slab mainly influence the frame beams which are directly connected with it，and almost has no influence on the frames beams which are not directly connected with it． Based on this，the causes and composi- tions of beam axial forces under the action of sagging and hogging moments were studied respectively． Then a de- sign recommendation considering the influence of monolithic slab on frame beam was proposed to improve the reali- zation effect of strong column weak beam．