The time-energy properties of high-order harmonic generation （HHG） are calculated for a linearly polarized 7- fs laser pulse with different carrier-envelope phases （CEPs）. The quantum trajectory paths that contribute to an as （1 as=10^-18 s） pulse in HHG are identified. The laser-duration dependence and the CEP dependence of HHG energy property are investigated. The study shows that an as extreme ultra-violet （XUV） pulse can be selected from HHG spectrum near cut-off energy with a bandpass optical filter. The theoretical prediction of the pulse duration is proportional to bandwidth. Analysis suggests that a measured narrowband as XUV pulse may consist of instantaneous shorter pulses each dependent on laser pulse duration, intensity, and CEP. These information can be used as references for producing, selecting, improving and manipulating （timing） as pulses.
