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Observations of very early multi-wavelength afterglows are critical to revealthe properties of the radiating fireball and its environment as well as thecentral engine of gamma-ray bursts (GRBs). We report our optical observationsof GRB 111228A from 95 sec to about 50 hours after the burst trigger andinvestigate its properties of the prompt gamma-rays and the ambient mediumusing our data and the data observed with {\em Swift} and {\em Fermi} missions.Our joint optical and X-ray spectral fits to the afterglow data show that theambient medium features as low dust-to-gas ratio. Incorporating the energyinjection effect, our best fit to the afterglow lightcurves with the standardafterglow model via the Markov Chain Monte Carlo (MCMC) technique shows that$\epsilon_e=(6.9\pm 0.3)\times 10^{-2}$, $\epsilon_B=(7.73\pm 0.62)\times10^{-6}$, $E_{\rm K}=(6.32\pm 0.86)\times 10^{53}\rm erg$, $n=0.100\pm 0.014$cm$^{-3}$. The low medium density likely implies that the afterglow jet may bein a halo or in a hot ISM. Achromatic shallow decay segment observed in theoptical and X-ray bands is well explained with the long-lasting energyinjection from the central engine, which would be a magnetar with a period ofabout 1.92 ms inferred from the data. The $E_p$ of its time-integrated promptgamma-ray spectrum is $\sim 26$ KeV. Using the initial Lorentz factor($\Gamma_0=476^{+225}_{-237}$) derived from our afterglow model fit, it isfound that GRB 111228A satisfies the $L_{\rm iso}-E_{\rm p,z}-\Gamma_0$relation and bridges the typical GRBs and low luminosity GRBs in this relation.

Multi-Wavelength Observations of GRB 111228A and Implications for the Fireball and its environment