  We present very early ground-based optical follow-up observations of
GRB~140423A, which was discovered by \emph{Swift}/BAT and by {\it Fermi}/GBM.
Its broadband afterglow was monitored by {\it Swift}/XRT and ground-based
optical telescopes from $T_0+$70.96~s to 4.8~d after the {\it Swift}/BAT
trigger. This is one more case of prompt optical emission observation. The
temporal and spectral joint fit of the multiwavelength light curves of GRB
140423A reveals that achromatic behavior is consistent with the external shock
model including a transition from a stellar wind to the interstellar medium
(ISM) and energy injection. In terms of the optical light curves, there is an
onset bump in the early afterglow with a rising index $\alpha_{\rm O,I} = -0.59
\pm 0.04$ (peaking at $t_{\rm peak}-T_0 \approx 206$~s). It then decays with a
steep index $\alpha_{\rm O,II} = 1.78 \pm 0.03$, and shows a steeper to flatter
"transition" with $\alpha_{\rm O,III} = 1.13 \pm 0.03$ at around $T_0 +
5000$~s. The observed X-ray afterglow reflects an achromatic behavior, as does
the optical light curve. There is no obvious evolution of the spectral energy
distribution between the X-ray and optical afterglow, with an average value of
the photon index $\Gamma \approx 1.95$. This "transition" is consistent with an
external shock model having the circumburst medium transition from a wind to
the ISM, by introducing a long-lasting energy injection with a Lorentz factor
stratification of the ejecta. The best parameters from Monte Carlo Markov Chain
fitting are $E_{\rm K,iso} \approx 2.14\times10^{55}$ erg, $\Gamma_0 \approx
162$, $\epsilon_e \approx 0.02$, $\epsilon_B \approx 1.7\times10^{-6}$, $A_\ast
\approx 1.0$, $R_t \approx 4.1\times10^{17}$ cm, $n \approx 11.0 \rm\ cm^{-3}$,
$L_0 \approx 3.1\times10^{52} \rm\ erg\ s^{-1}$, $k \approx 1.98$, $s \approx
1.54$, and $\theta_j > 0.3$ rad.
