With their increasingly wide application in large systems, embedded systems become more and more complicated. However, up to now there has been no mature methodology in embedded software design. In order to resolve this problem, much research has been performed. Designing embedded software based on models has become one of the research focuses. At present, the research on the design of model-based embedded software mostly focuses on theoretical models and simulations, rarely involving in model transformations and implementations of systems. In this paper, a new model-based design method of embedded software is presented for an OSEK compatible operating system-SmartOSEK OS. It emphasizes the transformation from problem domain to implementation domain. First, we use Ptolemy II to model and simulate in problem domain. Then, the transformation from problem domain to implementation domain is carried out. We have developed a suit of tools to support the whole developing process from modeling to code generation. It has been proved that higher productivity and quality can be achieved by using this method.