The invention discloses an online safety and stability evaluation method, device and system for a power system. The method comprises the steps that a current state safety and stability evaluation index is acquired, the current state safety and stability evaluation index comprises a ground state safety evaluation index and a ground state stability evaluation index, and the ground state safety evaluation index and the ground state stability evaluation index respectively comprise a plurality of second-level indexes; and the current operation state of the power grid is acquired calculating the value of each secondary index in the ground state safety assessment index and the ground state stability assessment index are calculated , and ground state safety assessment and ground state stability assessment are carried out based on the calculated value of each secondary index. According to the method, the current operation state of the power grid and the safety and stability evaluation index under the anticipated fault are divided into two categories according to the safety and the stability respectively, so that regulation personnel can identify the safety and stability characteristics of the power grid more effectively.