As urban green spaces, parks can provide rich cultural ecosystem services (CES), enhancing the well-being of those living in urban areas. Understanding how people perceive the CES supplied by parks and identifying differences with their supply is crucial for decision-makers and urban planners. In this study, we conducted a quantitative assessment of CES by combining an expert field investigation of parks with visitor questionnaires in the Three Hills and Five Gardens area of Beijing, China. Our assessment system comprised five categories of CES (landscape aesthetics, historical heritages, education, recreation, health and fitness) and eleven indicators. We identified differences between CES supply and perceptions and noted that such discrepancies additionally vary by CES type. We found that multiple CES are interdependent and interwoven. In addition, we discovered that perceptions of historical heritages service are particularly dependent on supply, while perceptions of health and fitness service are relatively independent of the supply. We explored the reasons behind these differences, finding that the visibility and prestige of historical heritages as well as the positioning and the overall condition of parks can affect visitors' perceptions. Our assessment can be used to guide the optimization of parks so that they may provide higher-quality CES for the public.