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 \sim 0.36 pc on the eastern side and \sim 0.46 pc on the western side , and the densities also show asymmetry , of \sim 0.015 cm ^ { -3 } on the east to \sim 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 .