During retrofitting or even in newly constructed RC flat slabs, near the columns, openings are provided. The existence of those openings reduces a significant amount of concrete and reinforcement bars responsible for punching shear resistance at the slab-column connection. This reduction interrupts the structural functionality of the slab-column connection. So, it is important to determine the reduction in strength due to the existence of the opening and restore its capacity to withstand the load imposed on it. Nowadays, due to its many advantages, carbon fiber reinforced polymer strengthening is becoming a popular strengthening technique. This study will numerically investigate the effect of opening on a flat slab and the use of the CFRP strengthening mechanism to restore the load-carrying capacity of the RC flat slab. ANSYS, the finite element analysis software, was used to perform a structural 3D non-linear finite element analysis for RC flat slab models. The accuracy of the finite element analysis (FEA) method was verified using experimental test results in the literature. To investigate the effect of openings on flat slab orientation, the number and shape of openings were used as parametric variables. Also, for the effect of CFRP strengthening, the type of CFRP, CFRP sheet fiber orientation, and CFRP bar orientation were used. Twenty-eight slabs with openings and one slab without an opening (control slab), a total of twenty-nine slabs, are examined for this study. The dimensions of the slabs are 1800mm×1800mm× 130mm. For this study, opening sizes of 75mm×75mm, 75mm×150mm, and 150mm×150mm were used. From slabs with openings, twenty-four of them are strengthened using CFRP strengthening. From the analysis, it is observed that providing an opening to the RC flat slab reduces up to 21.98% and 33.62% of the punching shear and energy dissipation capacity of the slab, respectively. To recover this loss, CFRP sheets and CFRP bars were used. Up to, 41.02% punching shear strength and 68.05% energy dissipation capacity increase were obtained by using the CFRP sheet. Up to 11.08% and 30.21% punching shear and energy dissipation increase were obtained by using CFRP bar respectively. Also, up to 30.98% punching shear strength and 90.41% energy dissipation capacity increases were obtained by using the combined CFRP sheet and CFRP bar. For the strengthening, it is observed that using combined CFRP sheet and CFRP bar strengthening and CFRP bar strengthening are more effective. Keywords: - Openings; CFRP; ANSYS; Finite Element Analysis; punching shear;