This paper studies the impact of Shanghai—Kunming high-speed railway on the tourism economy of cities along the line from 2007 to 2019 and its mechanism. Firstly, a multi-period double-difference method was used to assess the promotion effect of the opening of high-speed rail on the tourism economic development of cities along the route. Secondly, through the dynamic effect model, the long-term economic effect of high-speed rail construction is discussed, and the results show that the effect persists. Finally, the intermediary effect model and Bootstrap test were used to analyze the mechanism of high-speed rail, and it was found that tourism scientific and technological innovation had the highest proportion of intermediary effect in promoting tourism economy by high-speed rail, which was superior to tourism industry structure and traffic development. The overall results show that the opening of high-speed rail has a positive and lasting impact on the tourism economy of the cities along the line. With the passage of the construction cycle, tourism scientific and technological innovation has gradually become a key way to achieving high-quality tourism economic development.