Abstract: This paper takes aerospace precision instruments aees a typical application scenario and aims to develop materials with ultra-high modulus and low coefficient of thermal expansion. It designs the particle size distribution of ultra-high volume fraction SiCP/Al composites and optimizes their performance. A dual-particle size distribution method is adopted, and different-sized silicon carbide (SiC) particles are mixed with 6061 aluminum alloy matrix through mechanical powder mixing. This study systematically investigates the influence of the particle size of coarse particles in the particle size distribution on the mechanical properties and linear expansion coefficient of composites with different SiC volume fractions. The research results show that as the volume fraction increases, the linear expansion coefficient of all formulations of composites monotonically decreases. For composites with a volume fraction of 70%, increasing the particle size of coarse SiC further reduces the linear expansion coefficient. When the particle size of coarse particles is greater than 100 μm, although the bending strength of the 70% volume fraction composites decreases, their bending modulus, micro-yield strength and other mechanical properties are superior to those of the 65% volume fraction composites, demonstrating ideal comprehensive performance. When the particle size of coarse particles is less than 100 μm (73 μm), the 70% volume fraction composites have a narrower distance between coarse particles, causing fine particle agglomeration and a decrease in density, resulting in inferior mechanical properties compared to the 67% volume fraction samples. The final determined optimal formulation is a 70% volume fraction of 123 μm and 11 μm SiC particles in a 3:1 mass ratio, with a corresponding material bending strength of 379.61 MPa, bending modulus of 264.34 GPa, and linear expansion coefficient of 7.10×10-6 K-1.