A method of measuring excited modes in a fiber based on spatially and spectrally resolved measurement was adopted. According to the expression of the group delay of the fiber modes, the derivation processing of the group delay difference of transverse modes leading to the spectral interference was given as well as the corresponding simulation and analysis. The measurements for a standard communication single-mode fiber and a double-clad large-mode-area fiber were carried out. By using the single-mode fiber and optical spectrum analyzer to measure the spectral interference signal, the spectral interference signal by the Fourier transform was analyzed, and the distributions and power fractions of transverse modes were offered. The results show that the method of measuring the fiber modes based on spatially and spectrally resolved measurement can determine distributions and relative power levels of transverse modes in the fiber. When the high-order modes are weak compared with the fundamental mode, the interference between two different high-order modes can be ignored.