The ability to process multiple product types is an important criterion for evaluating the flexibility of a manufacturing system. The system dynamics of a multi-product system is quite distinct from that of a single-product system. A modeling method for the multi-product system is proposed based on dynamic systems and flow conservation. Based on the model, this paper places its emphasis on the analysis of a two-machine-one-buffer system with two product variants. The system performance measure of a multi-product system is proposed based on production orders. The system performance of two-machine-one-buffer systems is discussed in full details. The conditions for the system achieving the best performance are derived. Finally, several numerical experiments are conducted to validate the propositions on two-machine-one-buffer system.