A coherent spectrum analysis scheme based on a coherent receiver is proposed. The coherent receiver is used to perform in-phase and quadrature (IQ) mixing and balanced detection of the signal under test and optical local oscillator. The power of the signal under test is calculated according to the optical heterodyne principle, and the spectrum of the signal under test is reconstructed according to the mapping relationship between the calculated power and the corresponding frequency of the optical local oscillator. The experimental results show that the resolution is above 12.5 MHz when the sweeping rate of the optical local oscillator is in the range of 80-200 nm/s. The proposed spectral analysis scheme has an ultrahigh resolution and can be used to rapidly extract the fine spectral structure of ultradense wavelength division multiplexing systems.