A adjusting method of optically plane-concave laser resonator based on Gaussian beam is presented when the pump source is absent.A parallel incidence Gaussian beam through a lens system is used to hit on the concave mirror and the plane mirror of the plane-concave resonator.The importing of the lens system is to improve the adjusting precision of the beams reflected from the concave and the plane mirror by causing one beam size similar to another,because it can not get similar beam size reflected when a parallel incidence Gaussian beam is directly hit on the concave and the plane mirrors of the resonator.In the experiment,a He-Ne laser with waist of 0.6 mm is utilized to adjusting a plane-concave resonator with a concave mirror in a radius of 50 mm at measuring distance of 889 mm.The beam sizes reflected from the concave mirror and the plane mirror are 48 mm and 5.1 mm,respectively.The angle adjusting precision of the plane-concave resonator is 3.18′.TEM₀₀ mode can be abtained by pumping the resonator module that has been adjusted with a LD.The results show that the adjusting method is simple and flexible,and can be used to separate the laser resonator and the pump source module.