In order to improve the measuring precision of spectral line by spectrometer, the parameters of angle dispersive power, line dispersive power and resolving power of spectrometer with grating scanning controlled by computer have been calculated theoretically, and then a method has been put forward to improve the linewidth measurement by two united homogeneous spectrometers. The principle of measuring laser linewidth by two united spectrometers is analyzed theoretically, and the linewidth of He-Ne laser is measured. The experimental results shows amplification factor of full width at half maximum (FWHM) of He-Ne laser spectral line is 3.38×105, and 5 order magnitude of the line width measurement is enhanced at least by the two united homogeneous spectrometers than only one.