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We reported the optical observations of GRB 121011A by 0.8-m TNT telescope atXinglong observatory, China. The light curve of optical afterglow shows asmooth and featureless bump during the epoch of $\sim$130 sec and $\sim$5000sec with a rising index of $1.57\pm0.28$ before the break time of $539\pm44$sec, and a decaying index of about $1.29\pm0.07$ up to the end of ourobservations. Meanwhile, the X-ray light curve decays in a single power-lawwith a slop of about $1.51\pm0.03$ observed by $XRT$ onboard ${\rm} Swift$ from100 sec to about 10000 sec after the burst trigger. The featureless opticallight curve could be understood as an onset process under the external-shockmodel. The typical frequency has been below or near the optical one before thedeceleration time, and the cooling frequency is located between the optical andX-ray wavelengths. The external medium density has a transition from a mixedstage of ISM and wind-type medium before the peak time to the ISM at the laterphase. The joint-analysis of X-ray and optical light curves shows that theemission from both frequencies are consistent with the prediction of thestandard afterglow model without any energy injections, indicating that thecentral engine has stopped its activity and does not restart anymore after theprompt phase.

Optical light curve of GRB 121011A: a textbook for the onset of GRB afterglow in a mixture of ISM and wind-type medium