Despite the fact that China is becoming increasingly urbanised, some Chinese cities have witnessed a loss of urban population. In recent years, with the extensive expansion of the Chinese High-Speed Railway (HSR) network, the reduction in travel time has greatly increased the mobility of people across cities, thus affecting the spatial redistribution of the urban population in China. An important question arises: for cities experiencing population loss, when they were connected to the HSR network, did the HSR further aggravate this urban shrinkage? Using cities experiencing population loss as a sample, the paper provides a quantitative assessment of population loss in HSR-served cities. A Difference-in-Difference (DID) method was adopted to investigate whether the HSR aggravated population loss in shrinking cities. On the whole, empirical results show that the phenomenon of urban shrinkage in China was further aggravated by the HSR. Furthermore, the results unveil the dynamic impact of the HSR on shrinking cities, and multi-stage DID results reveal that the impact of the HSR on urban population may have a certain time lag effect. The HSR has significantly contributed to the population loss of shrinking cities four to five years after it first started operating. The paper highlights that the HSR does not always yield growth opportunities; on the contrary, for cities suffering from population decline, the HSR may bring a serious challenge.