The black hole X-ray transient XTE J1550-564 has 
undergone a strong
outburst in 1998 and two relativistic X-ray jets have 
been detected
years later with the Chandra X-ray observatory; the 
eastern
jet was found previously to have decelerated after its 
first
detection. Here we report a full analysis of the evolution 
of the
western jet; significant deceleration is also detected in 
the
western side. Our analysis indicates that there is a 
cavity outside
the central source and the jets first traveled with 
constant
velocity and then were slowed down by the interactions 
between the
jets and the interstellar medium (ISM). The best fitted 
radius of
the cavity is about 0.31 pc on the eastern side and 
about 0.44 pc on
the western side, and the densities also show 
asymmetry, of
about 0.034/cm^3 on the east to about 0.12/cm^3 on 
the
west. The best fitted magnetic fields on both sides are 
about 0.5
mG. Similar analysis is also applied to another 
microquasar system,
H 1743-322, and a large scale low density region is also 
found.
Based on these results and the comparison with other 
microquasar
systems, we suggest a generic scenario for microquasar 
jets,
classifying the observed jets into three main categories, 
with
different jet morphologies (and sizes) corresponding to 
different
scales of vacuous environments surrounding them. We 
also suggest
that the accretion disk winds and/or continuous jets 
may be responsible for 
creating these
cavities. Therefore X-ray jets from microquasars provide 
us with a
promising method of probing the environment of 
accreting black
holes.