To explore the shear performance and the corresponding mechanisms of Carbon Fiber Reinforced Polymer (CFRP) strengthened Reinforced Concrete (RC) shear walls, a three-dimensional numerical model based on the Hashin damage criteria that captures the CFRP-concrete interface debonding behaviors was developed. Using the proposed model, the influences of shear span ratio, CFRP ratio and wrapping method on the shear capacities of the CFRP-strengthened RC shear wall were investigated. It is found that: (1) the external CFRP strips effectively mitigate the development of the shear primary cracks; (2) the increasing shear span ratio reduces significantly the shear contribution provided by CFRP strips on the CFRP-strengthened RC shear walls; (3) the shear contribution of CFRP is not linearly dependent on the number of CFRP layers. From qualitative to quantification analysis, the influence coefficient of shear span ratio and CFRP layer was introduced based on the numerical calculations. Furthermore, writing in the form of the American Code (ACI 440.2R-17), a calculation formula characterizing the shear contribution of CFRP was established. By comparing with the experimental data, it is noticed that the proposed formula gives more accurate descriptions on the influence of shear span ratio, CFRP ratio and wrapping method on the shear contribution of CFRP. The average absolute error between the prediction results and the experimental results is 8.0%, thus, verifying the effectiveness of the proposed calculation method.