Recently , Rastall gravity is undergoing a significant surge in popularity .
We obtain a power-law total mass-density profile for the inner region ( within several effective radius ) of early-type galaxies ( ETGs ) from the space-time structures which are described by the static spherically-symmetric solutions of Rastall gravity under the assumption of perfect fluid matter .
We find that in the inner region of ETGs , the Rastall dimensionless parameter \beta = \kappa \lambda determines the mass distribution .
We then use 118 galaxy-galaxy strong gravitational lensing systems to constrain the Rastall dimensionless parameter \beta .
We find that the mean value of \beta for total 118 ETGs is \beta = 0.163 \pm 0.001 ( 68 % CL ) with a minor intrinsic scatter of \delta = 0.020 \pm 0.001 .
Our work observationally illustrates the physical meaning of the Rastall dimensionless parameter in galaxy scale .
From the Newtonian approximation of Rastall gravity , we also find that an absolute isothermal mass distribution for ETGs is not allowed in the framework of Rastall gravity .