Abstract: The type-A aluminum alloy city railway car-body is the most typical vehicle structure widely used in city railway vehicle market. Its underframe bears the functions of hoisting components，transmission power and so on，featuring high technical requirements，difficult processing and manufacturing. Through an analysis of the technical requirements for the underframe processing，the design process method and the planning processing procedure，the detailed manufacturing process of the underframe processing is fully displayed. Its plane tolerance distribution method，the measurement compensation technology，the multi-process drilling cycle and so on can provide reference for the manufacture of same products，and serve as the standardized operation of the underframe processing.