Based on theoretical model of beat frequency using two independent lasers, beat frequency linewidths of Lorentz and Gaussian linetypes changing with different attenuation values are obtained theoretically. It is significant that smaller attenuation value should be chosen to calculate laser linewidth if laser linetype is unknown previously and larger attenuation value for known laser linetype. In the experiment of beat frequency using two external cavity diode lasers (ECDL), linewidth ratio is measured under typical attenuation values, which can be used to judge linetype of laser. Using the method above, linewidth of laser is achieved with an error about 0.23% compared with its calibration data, therefore linewidth measurement precision of laser is enhanced. The method is still applicable to other linetypes analysis.