Motivated by the in-depth economic globalization and the deepening of China’reform and opening， R&D cooperation with large foreign firms has gradually become an important way for domestic firms to participate in the innovation-driven development strategy. This paper analyzes the factors influencing the endogenous selection of international cooperative R&D of downstream firms by constructing a negotiation game model under the situation of intermediate inputs trade. This paper finds that only when the R&D spillover effect is relatively small，will the downstream firms choose to conduct international R&D cooperation. However，if the R&D spillover effect is too small，the international cooperative R&D activities of downstream firms will lead to a decline of social welfare. It is also found that the enhanced monopoly of the upstream market and the policy of final product trade liberalization would promote international cooperative R&D of firms in the downstream of the industrial chain.