Composite rectangular tubes were widely used in aerospace and related fields and often served as critical load-bearing and functional components，imposing stringent requirements on dimensional accuracy and surface quality that conventional pultrusion and filament-winding processes struggle to meet.Integrating the advantages of compression molding and soft-mold（compliant-tool）expansion，this paper investigated a high-precision molding-thermal-expansion process for fabricating composite rectangular tubes and detailed the workflow of material selection，tooling design，and manufacturing.In the proposed scheme，the matched female and male dies are made of Invar steel，while the core mandrel is aluminum alloy；by exploiting the large mismatch in their coefficients of thermal expansion，parts with high-quality inner and outer surfaces are obtained，and the dimensional accuracy meets the specified targets.This work offers a viable approach for the molding and fabrication of similar composite rectangular tubes.