Abstract: A new method for estimating inter-harmonics spectrum based on the improved cumulants-root MUSIC (IC-RMUSIC) algorithm is presented in this paper. The four-order cumulants is used to restraint high-order white noise and Gaussian noise. To overcome the disadvantage of low-frequency resolution of the traditional four-order cumulants, the diagonal slice of four-order cumulants is transformed into the linear combination of the modified covariance matrices to guarantee the high resolution of algorithm. In addition, by constructing MUSIC polynomial, the noise suppressing capability of this algorithm is further enhanced. Experimental results show this method not only has high frequency resolution, but also can effectively suppress noise, and can get the spectral estimation of harmonics and inter-harmonics in noisy environments.