We report an optical spectroscopy study on an iron-selenide single crystal of insulating Cs0.8(Fe1.05Se)2 with relatively high Néel temperature.The resistivity and magnetization measurements reveal a semiconductor behavior with TN =492 K.The sample has been examined in the temperature range from 10K up to 500 K by using infrared measurement.The low frequency optical measurement implies an insulating response with a major absorption near 0.31 eV.The double-peak structure in the mid-infrared region and the low-frequency phonon modes support the presence of the blocked anti-ferromagnetism.The infrared measurement reveals no signature of SDW-type energy gap formation below the anti-ferromagnetic-order temperature.