In order to study systematically the soft X-ray emission of Active Galactic Nuclei ( AGNs ) at medium to high redshifts , we have analyzed ROSAT PSPC and HRI data of QSOs at 0.26 \leq z \leq 3.43 selected from the second deepest ROSAT PSPC survey carried out in 1991-1993 by McHardy et al . ( 1998 ) . Our sample of 22 type 1 QSOs is nearly complete above a flux limit of 1.4 \times 10 ^ { -14 } { ~ { } erg~ { } cm ^ { -2 } ~ { } s ^ { -1 } } in the 0.1 - 2 { ~ { } keV } band . Of these , nine QSOs show long term ( \sim 2 { ~ { } yr } ) X-ray variability by a factor of 1.5 - 3.5 . Significant excess absorption above the Galactic column is seen in three QSOs . The soft X-ray photon index of the QSOs ranges from 1.4 to 3.7 . Three QSOs have steep soft X-ray spectra ( \Gamma _ { X } > 3.0 ) , one of which is a narrow-line QSO – a high luminosity version of narrow-line Seyfert 1 galaxies . The average photon index ( < \Gamma _ { X } > ) is 2.40 \pm 0.09 ( with a dispersion of 0.57 ) in the 0.1 - 2 { ~ { } keV } band . The average QSO spectra in four redshift bins flatten from an average photon index of < \Gamma _ { X } > \sim 2.53 at 0.25 \leq z \leq 1 to < \Gamma _ { X } > \sim 2 at 2 \leq z \leq 3.4 . The flattening of the average photon index can be understood in terms of the redshift effect of the mean intrinsic QSO spectrum consisting of two components – a soft X-ray excess and a power-law component . We have also studied optical spectra of 12 of the 22 QSOs .