The Ne-like argon 46.9 nm laser pumped by capillary discharge was investigated experimentally. An adverse influence of the inherent pre-pulse generated by Blumlein transmission line of the capillary discharge setup on arising and stable output of soft X-ray laser was observed. The destroying effect of the inherent pre-pulse on uniformly pre-discharging the capillary has been calculated and proved theoretically. Actually, uniformly pre-discharging gas filled in capillary before the current main-pulse is the key condition of laser arising. In order to decrease the influence of the inherent pre-pulse, a method of installing switch and capacitor for insulating the inherent pre-discharge pulse has been proposed. The experimental results show that the method can insulate the inherent pre-discharge pulse efficiently and make the discharge setup steadily operate. On the basis of this the laser spike of Ne-like argon 46.9 nm has been observed. And the stability of laser output has been realized. The range of variety of laser intensity is less than 28%.