  Two relativistic X-ray jets have been detected with the Chandra X-ray
observatory in the black hole X-ray transient XTE J1550-564. We report a full
analysis of the evolution of the two jets with a gamma-ray burst external shock
model. A plausible scenario suggests a cavity outside the central source and
the jets first travelled with constant velocity and then are slowed down by the
interactions between the jets and the interstellar medium (ISM). The best
fitted radius of the cavity is ~0.36 pc on the eastern side and ~0.46 pc on the
western side, and the densities also show asymmetry, of ~0.015 cm$^{-3}$ on the
east to ~0.21 cm$^{-3}$ on the west. Large scale low density region is also
found in another microquasar system, H 1743-322. These results are consistent
with previous suggestions that the environment of microquasars should be rather
vacuous, compared to the normal Galactic environment. A generic scenario for
microquasar jets is proposed, classifying the observed jets into three main
categories, with different jet morphologies (and sizes) corresponding to
different scales of vacuous environments surrounding them.
