The invention discloses a rotary inertia identification method for a permanent magnet synchronous motor AC servo system, which adopts a speed command generator, a motor and a load, and comprises the following steps that: the permanent magnet synchronous motor AC servo system generates enable which has the rotary inertia identification function, and a user sets the rotary inertia identification duration; the speed command generator generates a uniform acceleration command of a first acceleration in the forward direction to make the motor accelerated to a first rotating speed; the speed commandgenerator generates a uniform acceleration command of a second acceleration in the forward direction to make the motor accelerated to a second rotating speed; the speed command generator generates a uniform acceleration command of a third acceleration in the forward direction to make the motor reduced to the first rotating speed; and the speed command generator generates a uniform acceleration command of a fourth acceleration in the forward direction to make the motor reduced to a zero rotating speed, and a forward-direction rotary inertia identification cycle is completed. The method ensureshigh identification precision in the rotary inertia identification process.