The system modal parameters identification under ambient excitation becomes the focus of the structural health monitoring. The improved ITD method was put forward based on stochastic subspace method considering ITD method has worse identification accuracy. Through the agency of four-story steel frame model experiment,it was proved that the improved ITD method was more accurate for the model of damping ratio and frequency identification than the ITD method. The improved ITD method could be applied to structural modal parameter identification under ambient excitation. Compared with the SSI method, the precision has not reduced, simultaneously reduced the computing time, It could provide the possibility for the application of this method to real-time monitoring of the structures.