Considering the limitation of the DH rule when using it to establish coordinate systems for robots, an auxiliary coordinate method was proposed as the extension of the traditional DH rule. The details about how to construct the auxiliary coordinate and how to use the auxiliary coordinate method were stated comprehensively. By the proposed method, any point of a link, not exactly in the joint of two links as usual, can be chosen as the original point of the auxiliary coordinates. The analytical results based on the auxiliary coordinate method were compared with the numerical simulation and it was found that they were in good agreement. It is shown that the method presented is more accurate and flexible when applied to complex robot systems while one or more components are not straight link, for example in L-shape, or t a relative rotation of one link about the axis of the connected previous one.