A new weight-fusion algorithm was proposed for frequency estimation of multi-sinusoids with the known frequency-ratio and the same length (Multi-Sinusoids-KFRSL), due to its significant research value and disadvantages of its present frequency estimation methods. According to the known frequency-ratio, the frequency-ratio amend matrix is given, in order to make spectrums of the Multi-Sinusoids-KFRSL as the same as those of multi-sinusoids with the same frequency and length. For the phase-incoherent problem of the Multi-sinusoids-KFRSL, the phase compensating matrix is created to make phase coherent and noise eliminate. Next, spectrums of the Multi-Sinusoids-KFRSL are weight-fused by the phase compensating matrix to turn almost the same as the spectrum of the phase-coherent sinusoid signal, which is as the same length as the Multi-Sinusoids-KFRSL. Consequently, precise frequency estimation can be obtained through spectral peak searching of the weight-fusion spectrum. Algorithm analyses and simulation results show that, compared with the present methods, the proposed algorithm works better in term of precision, calculation, noise immunity, significant theoretical and practical value.