A high-density computer system comprises: a chassis, having a main backplane, and a main control circuit and a plurality of main backplane slots installed and formed on the main backplane respectively; at least one expansion card module inserted and installed in the chassis, and the expansion card module includes a sub-backplane having a sub-control circuit and a plurality of second sub-backplane slots, and an end of the sub-backplane has a first sub-backplane slot corresponding to the main backplane slot, and the first sub-backplane slot and the sub-control circuit and the second sub-backplane slots being electrically coupled; a plurality of CPU cards installed on the expansion card module, each being inserted to the second sub-backplane slot, and the CPU card having a CPU, such that the computer system can have a powerful modular assembling function to enhance the economic benefits and product competitiveness.