Abstract: Theoretical analysis and working principle of the delay self-heterodyne method for measuring laser line width are presented in detail. Based on the deduced power spectral density function, different lengths of fiber delay line are simulated and the influence on measuring results is analyzed. A new method is proposed to solve the arithmetic solution of the ultranarrow laser line width when the fiber delay line is too long for practical application. After that, a typical analytical solution under certain conditions is given. When laser length is 300 m, the different methods to solve laser spectrum are given and proved by experiment.