In order to explore the deformation performance of the prestressed partially clad steel composite beams under vertical loading, a vertical two-point loading test is carried out on 12 prestressed partially clad steel composite beams to explore the deformation performance such as crack development, deflection and ductility. In this paper, the fractal dimension theory is used to analyze the crack development of each specimen. Based on the Euler beam theory and Timoshenko beam theory, the flexural equations of beams are derived respectively, and then the mid-span deflection of beams is calculated. The displacement ductility coefficient, section curvature ductility coefficient and energy ductility coefficient of prestressed partially clad steel composite beams are calculated. Influence factors of three ductility factors are analyzed by gray correlation analysis. The results show that the cracks develop most fully when the partially clad steel composite beams with a certain degree of pre-compressive stress are applied. The deflection equation based on the theory of Timoshenko beam and the calculation value of mid-span deflection are closest to the real value. The section area is the most important factor affecting the ductility coefficient of displacement and the ductility coefficient of curvature, and the anchoring form of section steel is the most important factor affecting the ductility coefficient of energy.