The invention discloses a method for evaluating the control performance of a PID (Proportion Integration Differentiation) loop based on subspace identification. The method comprises the following steps of: effectively segmenting a controller output signal u and a process output signal y according to the loop opening and closing states of a PID loop controller and the upper and lower limit constraints of the controller output signal u; solving a random system model by using a subspace identification algorithm; calculating a moment performance index and a performance index sequence at the reach of steady-state time according to the sampling time of the controller output signal u and the process output signal y, set PID loop steady-state time and desired time; and detecting oscillation in the loop according to a model obtained by using space identification and a self-correlation function of the process output signal y. The random system model is established by using the subspace identification, so that setting of a system order is avoided. By adopting the method, loop related information including 95 percent of steady-state time, 95 percent of dynamic time, overshoot and a peak moment is given by making full use of the model obtained by the identification. The performance index is calculated by using reference time set by a user, and is accordant with the practical situation of the PID loop.