power-law mass distributions in regions covered by the X-ray and lensing observations [@Humphrey10; @Koopmans06], we model the lens galaxy with a power-law mass distribution ($\rho \sim r^{- \gamma}$). The main idea of our method is that formula for the Einstein radius in a power-law lens expresses as $$\label{Einstein} \theta_E =   4 \pi
\frac{\sigma_{ap}^2}{c^2} \frac{D_{ls}}{D_s} \left(
\frac{\theta_E}{\theta_{ap}} \right)^{2-\gamma} f(\gamma),$$ based on which the ratio of angular-diameter distances between lens and source ($D_{ls}$) and between observer and source ($D_{s}$) can be obtained $$\label{Einstein}  \frac{D_{ls}}{D_s} =   \frac{\theta_E} {4 \pi}
\frac{c^2}{\sigma_{ap}^2}  \left( \frac{\theta_E}{\theta_{ap}}
\right)^{\gamma-2} f(\gamma)^{-1},$$ $f(\gamma)$ represents a function of the radial mass profile slop [@Koopmans] and $\sigma_{ap}$ is the luminosity averaged line-of-sight velocity dispersion of the lens inside the aperture radius, $\theta_{ap}$ (more precisely, luminosity averaged line-of-sight velocity dispersion). It is obvious that combining $\sigma_{ap}$, $\theta_E$, $\theta_{ap}$ and $\gamma$ obtained from the observations will introduce the measurement of the distance ratio of $D_{ls}/D_s$. Current observational techniques allow the redshifts of the lens $z_l$ and the source $z_s$ to be measured precisely. Moreover, imaging and spectroscopy from the Hubble Space Telescope (HST) and ground-based observatories make it possible to derive two key ingredients for individual lenses: stellar velocity dispersion, high-resolution images of the lensing 