In He-Ne laser,since the 633 nm line has higher gain than the other 5 lines around its band,as far as the mode competition is concerned,it′s hard for the line with low gain (such as the 629 nm line) to emit.A spectral selection method in He-Ne laser by Fabry-Perot (F-P) etalon is discussed in this paper.By choosing the proper length and reflectivity of the F-P etalon,the line with high gain can be restrained while keep the weak one untouched.In this way,the line with weak gain can emit.The transmitting properties of F-P etalon in cavity are calculaled by Gaussian beam theory.