To clarify the specific impact paths among physical activity, activity of daily living, depression and quality of life among dementia patients. A descriptive, cross-sectional design. Dementia patients and their caregivers from five tertiary general hospitals and one dementia patients' club were recruited. A total of 216 valid questionnaires were collected from November 2018 - March 2019. Path analysis was performed by Mplus 7.0 to test the casual relationship among physical activity, activity of daily living, depression and quality of life. Most patients with dementia presented a low level of physical activity and suffered from impaired quality of life. The modified model presented a good model fit and revealed that physical activity had indirect positive effects on quality of life mediated by activity of daily living and depression. The results showed physical activity was a promising nonpharmacological method to improve the performance of activity of daily living and reduce depressive symptoms and then enhance the quality of life among dementia patients. The findings were beneficial to elevate the awareness of physical activity among patients and professionals. This study was helpful to understand how physical activity exerted effects on life quality of dementia patients. This study provided a new perspective for researchers to elucidate the causal relationship of variables among dementia patients. © 2019 John Wiley & Sons Ltd.