活动星系核(AGN)是活动星系的中心区域，具有许多不同于一般星系和恒星的特殊性质。尘埃消光对还原AGN本征光谱，进一步研究AGN的性质具有重要意义。观测发现，银河系星际空间的光学消光(Av)与9.7μm硅酸盐消光(△τ9.7)之比Av/△τ9.7约为18，在银心方向Av/△τ9.7约为9，而AGN中该值为6.4，仅为银河系的1/3，猜测可能是由于AGN中尘埃颗粒较大导致的。本文以硅酸盐尘埃为模型，利用Mie理论计算不同尺寸大小的尘埃的消光效率随波长的变化，得到Av/△τ9.7与尘埃半径a的关系。计算结果表明，Av/△τ9.7在a≈0.2μm附近达到峰值约5.6，后随a增大而减小；如果考虑碳尘埃对光学波段的消光，则Av/△τ9.7≈6.4时，a>0.2μm，大于银河系尘埃的平均尺寸0.1μm，可以解释AGN的观测结果。尘埃的凝聚增长或小尘粒被高能光子选择性地击碎都会使AGN的平均尘埃颗粒变得较大。

AGN is the central region of active galaxy, and it has many special features differing from those of normal galaxies or stellars. Dust extinction is crucial to recover the AGN intrinsic spectra. In the interstellar medium of the Milky Way, the radio of optical extinction (Av) and the silicate absorption optical depth(△τ9.7) Av/△τ9.7≈18; In the galactic center, Av/△τ9.7≈9, while in AGN, it is approximately 6.4, only one third of the Galactic value. We predict it may be the result from the size of dust. In this work, we calculate the extinction efficiencies of silicate dust as a function of wavelength over a wide range of dust sizes. We find Av/△τ9.7 peaks at a≈0.2μm, with a peak value ~5.6. In view of the additional contribution of carbon dust to the optical extinction, we argue that the observed low Av/△τ9.7 value of AGNs could be explained by dust of a>0.2μm. The large grains may be caused by preferential destruction of small grains by X-ray/UV photons and/or grain growth in the dense circumnuclear regions of AGNs.