The invention discloses an improvement method of a diffraction Gaussian beam analysis (DGBA) algorithm and an analysis method of field distribution in a quasi-optical network system. A plane is designed between a device for emitting radiation field and a reflecting mirror as a novel feed source; a discrete field value of the novel feed source is obtained by field sampling on the plane; the field value of a sampling point is obtained by measurement or simulation calculation; and the existing feed source field analytic expression of Gaussian approximate representation is replaced by the novel feed source discrete field in the DGBA algorithm. The improved filed distribution analysis method and the DGBA algorithm adopting the filed distribution analysis method are applicable to analysis of any field distribution in the quasi-optical network system and comprises field distribution of the traditional feed source or field distribution of signal regulating devices passing between the two reflecting mirrors, thus expanding the application scenes of the DGBA algorithm, being more accurate of the field value obtained by measurement and improving the practical application value of the DGBA algorithm.