The invention relates to a motor train unit brake lining wear loss estimation method based on on-line monitoring. The estimation method comprises the following steps of 1) monitoring the operation state of a motor train unit and the working state of a subsystem and transmitting collected train operation date to a ground receiving system; 2) storing the operation data and screening out speed, BC pressure, AC pressure and brake time duration during train operation from the received data through the ground receiving system; 3) according to the screened data, determining the equivalent friction distance of the pressure exerted on a brake lining and a brake disc at an nth time; 4) according to the equivalent friction distance and the BC pressure, determining the total wear loss of the brake lining at the nth time. The motor train unit brake lining wear loss estimation method based on on-line monitoring can help estimate the wear loss of every train brake lining relatively accurately according to the brake condition of every train and can prolong the service life of the brake lining under the premise of guaranteeing the brake performance of the brake lining, thereby avoiding waste and reducing the maintaining cost.