In this paper the transmission function properties of Fabry-Perot interferometer is analyzed by the convolution equation of transmission function when linewidth of input laser is not equal to zero. The full width of transmission function at half maximum is broadened and the visibility is reduced because of the instrumental linewidth and the periodicity of transmission function. The method for measuring laser linewidth using the fullwidth of transmission function at maximum is not strict
it must satisfy some limiting conditions. The limiting conditions for measuring laser linewidth using the full width transmission function at maximum are given.