S-transform have been applied to the detection of power system harmonics with its excellent time-frequency characteristics, but there are still some large defects, such as poor detection precision, large amount of calculation and test results affected by noises. To solve these problems, this paper has improved HS-transform of generalized S-transform, and proposes a fast algorithm of generalized HS-transform combined with the idea of the incomplete S-transform. Firstly, it detected each harmonic frequency according to the dynamic measurement of signal power spectrum envelope, then it made generalized HS-transform corresponding to the major harmonic frequencies, and the characteristic quantities of the resulting HS complex matrix determined the information of the harmonic amplitude and phase. Simulation results show that this method can quickly detect harmonic frequency and the specific parameters of the steady-state or transient harmonic signals. The prosed algorithm has some benefits to the detection with high accuracy and good real-time performance, and its sensitivity to noise has greatly reduced, which lays a good foundation for the practical application.