  Deep space laser ranging missions like ASTROD I (Single-Spacecraft
Astrodynamical Space Test of Relativity using Optical Devices) and ASTROD,
together with astrometry missions like GAIA and LATOR will be able to test
relativistic gravity to an unprecedented level of accuracy. More precisely,
these missions will enable us to test relativistic gravity to
$10^{-7}-10^{-9}$, and will require 2nd post-Newtonian approximation of
relevant theories of gravity. The first post-Newtonian approximation is valid
to $10^{-6}$ and the second post-Newtonian is valid to $10^{-12}$ in the solar
system. The scalar-tensor theory is widely discussed and used in tests of
relativistic gravity, especially after the interests in inflation, cosmological
constant and dark energy in cosmology. In the Lagrangian, intermediate-range
gravity term has a similar form as cosmological term. Here we present the full
second post-Newtonian approximation of the scalar-tensor theory including
viable examples of intermediate-range gravity. We use Chandrasekhar's approach
to derive the metric coefficients and the equation of the hydrodynamics
governing a perfect fluid in the 2nd post-Newtonian approximation in
scalar-tensor theory; all terms inclusive of $O(c^{-4})$ are retained
consistently in the equation of motion.
