Abstract: Based on the stochastic subspace method, using the property of matrix space, the projection from future data row space to past data row space is modified to an update mode for subspace tracking though QR decomposition. The left singular vector of the projection has been tracked by subspace tracking algorithm. Then the modal parameter is obtained by least square method. online modal parameter identification is Realized by output-data only. At last though an experiment, the efficiency and stability of the method has been proved by tracking the changing of the modal space and the modal parameter.