The imaging process of a strip-map mode synthetic aperture ladar (SAL) ultilzing quasi-monochromatic light illumination is investigated based on optics diffraction theory. The theoretical expressions of the matched filters and the corresponding azimuth resolutions for synthetic aperture imaging under several conditions are given. Results show that the matched filter in a strip-map mode SAL is not only expressed in transmitting wavelength,target distance and SAL azimuth coordinate,but also coupled with the transmitting beam and the configuration of the receiving optics. As a result,for a laboratory-scale SAL demonstration system utilizing Gaussian beam as the detecting light,the expressions of the matched filter for azimuth synthetic aperture imaging vary slightly as the target plane is positioned at the beam waist,inside Rayleigh range or outside multiple Rayleigh range of the transmitting beam. It is suggested that the target should be placed outside the multiple Rayleigh range in order to obtain the high azimuth resolution.