  In the framework of SU(2) singlet down type vectorlike quark model, we
present a comprehensive analysis for decays B-> X_s \gamma, B->X_sl^+l^- and
B-> K\pi. As for B-> X_s\gamma, we include the QCD running from the mass of the
down-type vector quark D to weak scale in the scenario with the D quark much
heavier than weak scale, and find that the running effect is small. Using the
recent measurements of B-> X_sl^+l^-, we extract rather stringent constraints
on the size and CP violating phase of z_{sb}, i.e., the tree level FCNC
coupling for b-> sZ. Within the bounds, we investigate various observables such
as forward-backward asymmetry of b->s l^+ l^-, the decay rates of B
->X_s\gamma, and B\to K\pi. We find that (1) The forward-backward asymmetry may
have large derivation from that of the SM and is very sensitive to z_{sb}, and
thus can be useful in probing the new physics.(2) By taking experimental errors
at 2\sigma level, both experimental measurements for B->X_sl^+l^- and B->K\pi
decays can be explained in this model.
