The invention discloses a method for estimating the access capability of an inertia-free power supply under system transient frequency stability constraints. The method comprises the following steps:determining system parameters related to frequency characteristics and a boundary of frequency stability; determining the type of a high-power disturbance fault; for different fault disturbance types,estimating the minimum number of started conventional units and the maximum output of a non-inertia power supply under various disturbance faults; and estimating the access capability of the inertia-free power supply under the frequency stability constraints of the system. The frequency stability of the system can be evaluated, the arrangement of the power grid operation mode and the power grid dispatching can be guided, and the power grid planning can also be guided.