Abstract: In order to realize the accurate analysis of mechanical properties of 3D printing fiber reinforced polymer (FRP) bridge components and promote the application of 3D printing FRP technology in bridge engineering, the key mechanical properties of 3D printing FRP were explored based on theoretical and experimental methods. Firstly, the hypothesis of printing filament continuity was proposed based on the spatial geometry characteristics of the meso-structure of printing FRP. Based on the hypothesis and the in-plane stress rotation axis model, the Young's modulus analysis and prediction model of 3D printing FRP under in-layer stress was constructed. At the same time, considering multiple failure modes of the material, a tensile strength prediction model under in-plane stress was established based on Tsai-Wu theory, and four different shear strength calculation modes were considered in this model. Secondly, considering the printing angle, filament width, and layer thickness, systematic testing and analysis of Young's modulus and tensile strength were designed to verify the accuracy of the above two theoretical models. The results show that there is an obvious negative correlation between printing angle and the two kinds of key mechanical properties. When the printing angle increases from 0° to 90°, the decrease range of Young's modulus is 65.48%-79.62%, and the decrease range of tensile strength is 50.99%-71.55%. The filament width has obvious influence on Young's modulus and tensile strength. These two kinds of key mechanical properties with 0.6 mm and 0.8 mm filament width are similar to each other, and both are significantly bigger than those with 0.4 mm filament width. The variation range of Young's modulus is 20.18%-49.27%, and the variation range of tensile strength is 27.53%-54.55%. The macro-scale failure results show that there are two types of failure modes, namely, printing filament fracture and printing filament separation. Additionally, the mechanism of the two types of failure modes and the influence mechanism of printing parameters on the key mechanical properties are revealed from the meso-scale. In conclusion, these two models established in this study provide theoretical support for the quantitative evaluation of the key mechanical properties of 3D printing FRP bridge engineering components.