Soft X-ray conversion efficiencies from 0.35μｍ laser-irradiated Au disk and C, Al, Cu foils at different laser intensity on the "Xingguang Ⅱ" high-intensity laser facility (energy ５—９０Ｊ，focal spot～２００μｍ，FWHM４００—８００ｐｓ）were obtained. A simple model was given to explain soft X-ray conversion efficiency from ０.３５μｍ laser-Al plasma interaction. In this model,bccause of heat conduction from the laser-heated spot, the conversion was very small at some low irradiancc limit, while at some high irradiance limit it was bounded by the energy lost in plasma blowoff. Consequently, at laser intensity(2±0.5)×１０１4Ｗ／ｃｍ2,the X-ray conversion efficiency reaches a maximum.