Chronic Obstructive Pulmonary Disease (COPD) is currently the third major cause of death–more than three million people died from it in 2019. The problem is exacerbated in Hong Kong due to an aging population. Given that COPD cannot be cured currently, immediate treatment is crucial. Breathing exercises(BR) are widely used to prevent COPD deterioration. Patients are advised to have BR at home to get sufficient treatment in time. Monitoring patients at home can help improve patient adherence and collect data on patient’s recovery progress from the rehabilitation team’s perspective. However, how to track if proper breathing is taken by a patient has remained challenging. In this work, we present BreathMentor, a smart-speaker-based breathing monitoring system that monitors breathing exercises. BreathMentor is both unobtrusive and preventive of privacy invasion so that it suits home care. BreathMentor converts the smart-speaker into an active sonar system that continuously perceives and analyses the changes in surroundings, thereby detecting the user’s respiration rate, deriving the breathing phases, and classifying whether the patient is practicing diaphragmatic breathing and pursed-lip breathing. BreathMentor further interacts with users through screens and speakers, allowing users to visualize their training and provide appropriate guidance.