Abstract: Ballast mixed with Tire-Derived Aggregate (TDA) is a novel method for improving the track bed, which has the advantages of promoting performance of ballasted track infrastructure and enhancing the utilization rate of solid waste. It has received significant attention from railway researchers. Existing studies mainly focus on the influence of different TDA content, while research on the impact of TDA particle shape is scarce. Particle shape is a crucial factor affecting the mechanical properties of granular materials. Therefore, it is necessary to study the mechanical properties of ballast improved by TDA of different shapes. Based on the basic classification method of particle shape in China's railway standards, different shapes of TDA particles (block, needle, and plate) were designed and processed. under the reference of optimal TDA content and particle size, 10% volume ratio and 9.5-19.5 mm size TDA were considered and triaxial tests were conducted under different confining pressures. The results show that improved ballast can maintain or enhance strength, reduce the initial elastic modulus and volumetric deformation, and decrease the modulus degradation and ballast breakage degree. However, due to the differences in particle deformation and contact behavior caused by the different shapes of TDA particles, the mechanical properties of the improved ballast show significant differences. Ballast improved by needle-shaped TDA has a lower modulus degradation degree, block-shaped TDA improved ballast has a stronger energy absorption capacity, and plate-shaped TDA improved ballast has a smaller expansion angle and volumetric deformation rate. Based on the proposed reasonable selection principle, considering the influence on the strength and deformation of ballast, it is found that needle-shaped TDA has the best improvement effect.