This paper investigates the impact of high-speed railways (HSR) on urban innovation levels by improving intercity accessibility. We employ prefecture city-level data within three megacity regions in China from 2009 to 2018. Using the number of invention patents granted as a proxy for the innovation level of a city, we find that HSR facilities significantly improve urban innovation levels through better regional intercity accessibility and that there is a diminishing effect as commuting time increases. The impact mechanisms of innovation improvement can be explained by an ever-increasing potential of interaction activities among talents and technology investment opportunities among cities. We contribute to the literature by highlighting the spatial attenuation impact of HSR on urban innovation levels as well as the underlying mechanisms. Particularly, talent interaction exerts a larger effect on urban innovation levels than technology investment opportunities. Moreover, we unravel the heterogeneous effects that more innovative cities and cities with double first-class universities gain more from the improved intercity accessibility brought by HSR. This research has policy implications that promoting HSR facilities improves innovation levels of cities with different resources.