As the spaces for dialogue between the past and the future, museums are essential to human well-being and social sustainability. Here, we collected data from 328 cities in 31 provinces of mainland China from 1980 to 2019 to investigate the changes in number and spatial inequalities of museums. The results showed that: (1) in mainland China, there were only 137 museums in 1980, and while this increased to 5626 in 2019, China still possessed only four museums per million people; (2) the increasing number of museums lagged behind the growth rate of both the population and economy at both the province and city level; (3) the Gini coefficient of museums per million people was only 0.27 in 2019, indicating relative equality of visiting opportunities among the provinces of China; (4) the Gini coefficients of per capita museums in some provinces were higher than that of the whole nation, with the highest ~0.6 in 2010 and 0.4 in 2017; (5) the economic competitiveness and human well-being of a city were promoted by an increased number of museums. We suggest that the central government of China should increase the number of museums in all provinces, while some provinces should pay more attention to the inequality in the distribution of cultural facilities among cities.