Abstract: The mutual influence effect from the corporate decision-making is more significant in the oligopolistic market structure. At this point, the corporate finance decisions and output decisions not only affect their own profits, and will affect their competitors and suppliers, that is, the strategic effect of the business decisions. In this paper, the dynamic game model of the upstream-downstream business decisions is established with considering product substitution and market fluctuations. A double particle swarm algorithm against that decision of nonlinear bilevel programming features is designed. Then the enterprise decision-making is analyzed by using computer simulation technology. It can be found that, firstly, product substitution has a "competitive effect" on downstream enterprises and "amplifying effect" on the decline of production and profit of upstream enterprises. Compared with debt financing limited liability effect, the "competitive effect" of the product substitution have a greater impact on production game and profit of the downstream enterprises. In addition, debt limited liability effect and debt bankruptcy effect together affect the impact of product substitution on the bankruptcy equilibrium critical point of downstream enterprises. Secondly, the impact of fluctuations in the market environment on production decision-making and financing decision is consistent with Wanzenried^[1], but the impact on profit of upstream and downstream enterprises is not consistent. Finally, when considering the two cases of debt financing and no debt financing, regardless of lower or higher product substitution, a higher degree of fluctuations in the market environment urge the financing enterprises to consider the strategic role of debt, and the phenomenon of "Prisoner's Dilemma" exists in downstream enterprises' financing decisions. Upstream firm's monopoly position in the market strengthens the impact of the "amplifying effect" of debt financing for downstream enterprises on its profit.

Key words: particle swarm optimization, strategic financing, cournot game, product substitution, the fluctuations in market environment