A method, a computing device, and a system for monitoring postures are proposed. The method includes the following steps. An image sequence captured on a monitored area including a monitored subject is received. A first stable value corresponding to a first stage of stability of the monitored subject is detected, where the first stable value is associated with a position corresponding to a posture of the monitored subject remaining consecutively stable within a first predetermined time period. A second stable value corresponding to a second stage of stability of the monitored subject is detected, where the second stable value is associated with a position corresponding to a posture of the monitored subject remaining consecutively stable within a second predetermined time period. The first stable value and the second stable value are compared to accordingly determine whether a posture of the monitored subject has changed.