  We revisited the extracting $\left|V_{ub}/V_{cb}\right|$ and $|V_{ub}|$
through calculating the ratios ${\cal B}(B^0\to D_s^{(*)+}(\pi^-,\rho^-)/ {\cal
B}(B^0\to D_s^{(*)+} D^{(*)-})$ in the perturbative QCD approach, which is
regarded as an effective theory in dealing with non-leptonic charmed $B$
decays. Utilizing this approach, we could calculate the form factors
effectively as well as non-factorizable and annihilation contributions. Within
the updated distribution amplitudes and the latest experimental data, we get
$\left|{V_{ub}}/{V_{cb}}\right|=0.083\pm0.007$, which favors a bit smaller
$|V_{ub}|$ compared with the averaged PDG value, but agrees well the
exclusively measured values. Furthermore, we predict the branching ratio of
$\bar B^0 \to D_s^-\rho^+$ $\sim$ $(2.7\pm 1.2)\times 10^{-5}$, which could be
measured in $B$ factories near future. In our calculation, the major
uncertainty is due to our poor knowledge of heavy meson wave functions. We also
comment that it is not trivial to generalize this approach to $B_s$ system,
primarily because both $\bar B_s^0 $ and $B_s^0$ can decay into the same final
states $D_s^+K^-$ and $D_s^+D_s^-$.
