An automated modelling approach of mechatronic multibody systems is presented in this paper. The proposed approach uses some object-oriented GUI modules to automatically generate the dynamic equations for different domains, solve them with numerical methods to obtain approximate solutions, and then evaluate the dynamic performances of the systems. By systematically defining an elementary linear graph and its general rules, the modules of mechanical parts and kinematic pairs can be modelled independently of special systems by the extensible elementary linear graph (EELG) method, and the member's dynamic equations can be derived by topology matrices operation. Some major advantages of this procedure are as follows: the combinations of mechanical components could be dealt with as an integrated member and directly assembled with other modules, the topology structure of individual members are described by elementary cutset and circuit matrices derived from the elementary linear graph, rotation vector is used to express angular variables for analysing rotation and translation with same linear graph; the function vertices, opening edges, and self-closed edges are first introduced to elementary linear graph of kinematic pairs modelling, multiport for special mechanical members and different ports for various energy domains are defined, and relation equations linking the ports are given for interdisciplinary domains, so that the modules could have the characteristics of reapplication and extensibility. For two typical cases, the approach carried out on a Modelica/Dymola software platform is proved feasible by comparing the results using the EELG method with those of the conventional approach.