  The equivalence principle is important in fundamental physics. The fifth
force, as a describing formalism of the equivalence principle, may indicate the
property of an unknown theory. Dark matter is one of the most mysterious
objects in the current natural science. It is interesting to constrain the
fifth force of dark matter. We propose a new method to use perihelion
precession of planets to constrain the long-range fifth force of dark matter.
Due to the high accuracy of perihelion precession observation, and the large
difference of matter composition between the Sun and planets, we get one of the
strongest constraints on the fifth force of dark matter. In the near future,
the BepiColombo mission will be capable to improve the test by another factor
of ten.
