The end deflection of the telescopic arm impacts print accuracy. To enhance stiffness, its structure was optimized using topology and size optimization. The U-shaped section of the telescopic arm was optimized using the variable density method in Ansys Workbench under two conditions, with the cross-section reconstructed based on topology optimization results. The wall thickness of the stiffener in the reconstructed cross-section was selected as the optimization variable, the mass of the three-section arm was minimized as the optimization goal, and the total deformation of the end of the three-section arm was less than 3.8 mm as the constraint, and the direct optimization method and screening method were used to optimize the size of the reinforcement. Remodeling with SolidWorks and performing a static analysis of the initial design of the telescopic arm and the optimized telescopic boom, the results showed that the total deformation at the end of the optimized telescopic arm was 1.14 mm smaller than the initial design under the most dangerous conditions, the deformation at the end of the telescopic arm has been reduced by 59.4%.The results show that the stiffness of the telescopic arm is optimized by topology optimization and size optimization.