The line shape of power spectrum of frequency-modulated laser with delayed self-heterodyne method is presented. Theoretical analysis is carried out and a mathematical expression is used to describe the line shape. According to the theoretical results, the line shape of frequency-modulated laser measured by delayed self-heterodyne method is the convolution of the natural line shape and the transfer function related with the modulation signal and the testing system. The dependence of line shape on the modulation frequency and the length of optical delay is analyzed based on the obtained expression. Experiments are carried out and the testing results verify the theoretical analysis results.