  Using the $B\to K_1$ form factors evaluated in the perturbative QCD approach,
we study semileptonic $B\to K_1(1270)l^+l^-$ and $B\to K_1(1400)l^+l^-$ decays,
where $K_1(1270)$ and $K_1(1400)$ are mixtures of $K_{1A}$ and $K_{1B}$ which
are $^3P_1$ and $^1P_1$ states, respectively. Using the technique of helicity
amplitudes, we express the decay amplitudes in terms of several independent and
Lorentz invariant pieces. We study the dilepton invariant mass distributions,
branching ratios, polarizations and forward-backward asymmetries of $ B\to
K_1l^+l^-$ decays. The ambiguity in the sign of the mixing angle will induce
much large differences to branching ratios of semileptonic B decays: branching
ratios without resonant contributions either have the order of $10^{-6}$ or
$10^{-8}$. But the polarizations and the forward-backward asymmetries are not
sensitive to the mixing angles. We find that the resonant contributions will
dramatically change the dilepton invariant mass distributions in the resonant
region. We also provide the angular distributions of $ B\to K_1l^+l^-\to
(K\pi\pi)l^+l^-$ decays.
