From thermodynamic theory of the lead zirconatetitanate(PZT) solid solution sy stem, we have got the basic equation between the dielectric constant εr a nd lattice distortion c/a in ferroelectric tetragonal phase region, and base d on the relative experimental data and logical deduction to physics image, we h ave established basic relations between εr and c/a. And then, from th e basic relations, we have studied in detail the physics image about lattice dis tortion c/a effect on the dielectric constant εr in ferroelectric tet ragonal phase region. A comparison with related experimental results indicates t hat the results of the basic relations are in good agreement with the conclusion s of the experiments. Finally, we have discussed the application range of the ba sic relations and the physics reason of relative warp out of application range. We would emphasize that the basic relations are essentially fitted to all perovs kite structure materials.