Iran's Look East and China's Belt and Road Initiative jointly pushed the two countries to sign the 25-year comprehensive cooperation agreement in 2021. This document has attracted widespread attention from the international community. The 25-year agreement is certainly conducive to the deepening of Sino-Iranian relations, but its role should not be exaggerated. The tortuous process of reaching the agreement partly explains the differences in need between China and Iran for the deepening of bilateral relations. This article aims to answer what challenges Sino-Iranian relations face and how to objectively understand the relationship between the two countries. The hypothesis of this article is that the 25-year agreement will not allow China and Iran to form a very close relationship. Based on literature analysis and field work, this article argues that low political mutual trust, partial competition of industrial products, different understandings of Iran's regional status, US sanctions against Iran, and their respective internal affairs have indeed restricted the deepening of China-Iran relations. To a large extent, the implementation of the 25-year comprehensive cooperation agreement has become a yardstick for measuring China-Iran relations. So far, the 25-year agreement has not brought any surprises to China-Iran relations.