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We present very early ground-based optical follow-up observations ofGRB~140423A, which was discovered by \emph{Swift}/BAT and by {\it Fermi}/GBM.Its broadband afterglow was monitored by {\it Swift}/XRT and ground-basedoptical telescopes from $T_0+$70.96~s to 4.8~d after the {\it Swift}/BATtrigger. This is one more case of prompt optical emission observation. Thetemporal and spectral joint fit of the multiwavelength light curves of GRB140423A reveals that achromatic behavior is consistent with the external shockmodel including a transition from a stellar wind to the interstellar medium(ISM) and energy injection. In terms of the optical light curves, there is anonset 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 asteep 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 doesthe optical light curve. There is no obvious evolution of the spectral energydistribution between the X-ray and optical afterglow, with an average value ofthe photon index $\Gamma \approx 1.95$. This "transition" is consistent with anexternal shock model having the circumburst medium transition from a wind tothe ISM, by introducing a long-lasting energy injection with a Lorentz factorstratification of the ejecta. The best parameters from Monte Carlo Markov Chainfitting are $E_{\rm K,iso} \approx 2.14\times10^{55}$ erg, $\Gamma_0 \approx162$, $\epsilon_e \approx 0.02$, $\epsilon_B \approx 1.7\times10^{-6}$, $A_\ast\approx 1.0$, $R_t \approx 4.1\times10^{17}$ cm, $n \approx 11.0 \rm\ cm^{-3}$,$L_0 \approx 3.1\times10^{52} \rm\ erg\ s^{-1}$, $k \approx 1.98$, $s \approx1.54$, and $\theta_j > 0.3$ rad.

GRB 140423A: A Case of Stellar Wind to Interstellar Medium Transition in the Afterglow