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There are some complicated interactions among different stages in multistage machining processes (MMPs), so it is important and necessary to explore the inherent mechanism of machining error propagation for improving product quality. With respect to this issue, a machining error propagation network (MEPN) of MMPs is proposed and its complexity is discussed. In order to establish this MEPN, the machining form feature (MFF) of the workpiece, machine tool, fixture, and cutting tool (i.e. machining element, ME) are defined as different network nodes. The associated relationship among different nodes is mapped by graph theory, according to the computer aided process planning (CAPP) information of the workpiece. Based on this, complex networks theory is introduced to analyse the characteristics of the established MEPN, and several measuring indices of error propagation at different nodes are proposed to quantify the interactions among different nodes. The synthesized propagation index (SPI) is then established to identify the key nodes and the SPI variation curve is used to quantify these key nodes' influences on the MEPN. Finally, a box part of a missile launcher is used to illustrate the proposed methods.