Disclosed is an online evaluation method of a safety margin of an operating reserve in a power system, which belongs to the technical field of power system operation and control. A post-fault steady operation state of the system is obtained by considering protection and through time-domain simulation of action of an automatic safety device, and active power sensitivity of the operating reserve for power transmission equipment and a power transmission corridor is calculated based on a direct-current (DC) power flow equation of the state. A maximum capacity of the operating reserve that is useful for actual control of a steady state frequency is obtained by solving an optimization function considering a controllable interval of the operating reserve, an overload limit of the power transmission equipment and a safety and stability limit of the power transmission corridor, and aiming at maximizing the operating reserve. A maximum controllable quantity of the steady state frequency is calculated in combination with power and frequency static characteristic coefficients, achieving online evaluation of a safety margin of the operating reserve taking faults into consideration.