Abstract: The procedure of fast identification algorithms of the time-varying modal parameter based subspace tracking is investigated in this paper. After the pretreatment to the updating input/output data, the problem of fast time-varying modal parameter identification changes to be a subspace tracking problem principally. Adopting the principal subspace tracking algorithms, the fast time-varying modal parameter identification algorithms can be deduced. Replacing PAST by NPI and API, two new versions of time-varying modal parameter identification algorithms are proposed. The performance of the fast modal parameter identification algorithms based on different principal subspace tracking algorithms are compared by applying them on a classical time-varying mechanical system of a moving distributed mass-cantilever beam and a 2-DOF spring-mass model.