The system design has been become an imperative part of the mechatronic system design with the increasing complexity. However, the current design platforms are all extended from the software modeling tools, which are essentially software-oriented and lack the support for the characteristics of complex mechatronics. Based on the analysis of key problems for the design of multi-domain complex mechatronics, a design-oriented modeling platform, M-Design, is developed for its design. First, the domain-independent fundamental semantics of SysML is represented in M-Design. Next, the domain-dependent design semantics is also modeled for various related domains of complex mechatronics. Finally, the transformation is designed to be conducted automatically from system design model to system simulation model to support the early simulation and find the irrational design schemes as early as possible. A case study is given to illustrate the proposed method.