To predict the flexural strengths of steel girder-concrete abutment connections in fully integral abutment bridges with perfobond connectors, this paper analytically and numerically investigates the ultimate flexural behavior of these connections. A nonlinear finite element model was first established and validated against experimental results. Subsequently, another 89 nonlinear finite element models with different connector quantities, connector arrangements, abutment widths, and shear-span ratios were studied. An analytical calculation method considering the moment contribution of perfobond connectors and the shear force distribution on perfobond connectors at ultimate was then proposed, verified against experimental and numerical results, and was compared with current calculation methods. Results show that, compared with current methods, the proposed method predicts the flexural strengths of the connections with better accuracy, with the average predicted-actual ratio being 0.96 and the coefficient of variation being 0.09. The flexural strengths of girder-abutment connections are significantly affected by girder embedded length, abutment width, and the quantity and arrangement of perfobond connectors. The flexural strengths are higher when connectors are set close to the girder end. When increasing the quantity of perfobond connectors, the flexural strengths of the connections could increase by 86% at most. With the same connector quantity, the flexural strengths could vary 20% with different connector arrangements.