Population size, age structure, and ageing of Chinese cities from 2000 to 2100

City-scale data on population size and age structure are essential for urban planning, public service provision, and disaster risk assessment. However, existing population datasets for China are mainly available at national or administrative-unit scales, while data for individual cities remain limited. Here, we developed a dataset of population size, age structure, and population ageing for 326 major Chinese cities from 2000 to 2100. Historical urban population data were derived by aggregating age- and sex-specific population grids within urban built-up areas after systematic calibration using census data. Taking 2020 as the baseline year, future populations were projected using the cohort-component method under five population scenarios based on the Shared Socioeconomic Pathways (SSPs), with projections provided at five-year intervals from 2025 to 2100. Historical backcasting showed that the mean absolute percentage errors (MAPE) for simulated total population and the population aged 65 years and older in 2020 were both below 10%, while the error in the simulated share of the population aged 65 years and older was below 1 percentage point. The dataset also showed good agreement with existing population datasets, and its future projections were broadly consistent with existing population projection products in terms of urban population size and regional ageing patterns. This dataset provides a basis for investigating urban population growth and shrinkage, population ageing, and regional demographic transitions in China, and can also support studies of urban disaster risk, public service demand, and sustainable urban development. Given China's rapid urbanization and continuing population ageing, the long-term demographic trajectories of its cities also provide useful evidence for understanding global urban demographic transitions and their social and environmental implications.This datasat contains age- and sex-specific population data for 326 Chinese cities represented by a consistent set of fixed city boundaries, together with the principal R modules documenting the data-production workflow. The dataset includes historical population data for 2000, 2010, and 2020; a simulated population for 2020; and population projections under five Shared Socioeconomic Pathway (SSP) scenarios from 2025 to 2100 at five-year intervals. Population values refer to the population within the fixed city boundaries used by this dataset. They should not be interpreted as the population of the entire prefecture-level administrative area.