systems. We take the fractional uncertainty of the Einstein radius at a level of 1%, which is reasonable for the future LSST survey to obtain high-resolution imaging with different stacking strategies for combining multiple exposures [@Collett16]. Following the Lens Structure and Dynamics (LSD) survey and the more recent Sloan Lenses ACS (SLACS) survey, we take the fractional uncertainty of the observed velocity dispersion at a level of 5%, which can be assessed from the spectroscopic data for central parts of lens galaxies. More importantly, it was shown that the power-law mass profile is still a useful assumption in gravitational lensing studies and should be accurate enough as first-order approximations to the mean properties of galaxies relevant to statistical lensing (lenses observed in different surveys with the following median values of the lens redshifts: $z_l = 0.215$ for SLACS, $z_l = 0.517$ for BELLS, $z_l = 0.81$ for LSD and $z_l = 0.456$ for SL2S [@Cao15Ap]). In our fiducial model, the average logaritmic density slope is modeled as $\gamma=2.085$ with 10% intrinsic scatter, the results from SLACS strong-lens early-type galaxies with direct total-mass and stellar-velocity dispersion measurements [@Koopmans09].

![**The lens redshift distribution of simulated strong lensing systems from future LSST survey.**](fig2.eps){width="0.9\linewidth"}

