The invention discloses a method for manufacturing an aluminum alloy section for an underframe of a rail transit vehicle body. The method comprises the following steps of: a, providing materials of a cast rod; b, melting the cast rod; c, casting the cast rod; d, performing homogenization treatment; e, performing surface treatment of the cast rod; f, heating the cast rod; g, heating a mould; h, performing extrusion production; i, performing online quenching; and j, ageing the section. In the method, parameters in the steps are strictly controlled; larger section and better comprehensive performance of the aluminum section can be obtained; the needs in using the aluminum section vehicle body are met; the manufacturing cost of the vehicle body is lowered, and the weight of the vehicle body is reduced; the shortcomings of large mass, great difficulty in maintenance, expensive auxiliary equipment and the like of a steel-structure vehicle body are eliminated; by adopting the method disclosed by the invention, the energy consumption is just 1 / 5 of that of stainless steel, the water consumption is just 1 / 5 of that of a stainless steel carriage, and the weight of the carriage is reduced by1 / 3; and moreover, the strength at non-proportional extension of the section can reach over 245 MPa, the tensile strength can reach over 290 MPA, and the percentage elongation after fracture can reach over 10%.