scatter in lens redshifts, we use a bin size less than 0.02 ($\Delta
z_l=0.02$). We also find that the choice of $\Delta z_l$ may play an important role in the accuracy and reliability of our test, which will be discussed in Section 4. More importantly, in order to guarantee the precision of GP reconstruction, the selected sub-sample should be large enough to yield enough statistics. In our simulated sample of strong-lensing systems, these criteria therefore allow us to assemble a sub-sample including 390 strong-lensing systems, with the lensing galaxies covering the redshift shell of $0.3<z_l<0.32$. **Fig. 2 shows the lens redshift distribution of galactic-scale lenses discoverable in forthcoming LSST surveys. It is apparent that, compared to the current surveys with the following median values of the lens redshifts: SLACS – $z_l =
0.215$, BELLS – $z_l = 0.517$, LSD – $z_l = 0.81$ and SL2S – $
z_l = 0.456$ [@Cao15Ap], the future LSST survey is particularly promising to discover more lenses covering the redshift range of $0.25 - 0.50$. Therefore, the thin shell of $0.3 < z_l < 0.32$ is a good statistical representation of the simulated population of lenses what the future LSST survey might yield.**

At each 