Disclosed herein is a cosmetic composition for preventing photoaging, containing an extract of Magnolia sieboldii as an active ingredient. Also disclosed is a cosmetic composition for reducing skin wrinkles containing an extract of Magnolia sieboldii as an active ingredient. The extract of Magnolia sieboldii can protect the skin against UV lights and skin damage by suppressing skin cell death and cell DNA damage, and can reduce skin wrinkles by promoting the biosynthesis of collagen and suppressing the synthesis of collagenases. Also, the cosmetic composition containing the extract of Magnolia sieboldii has an excellent effect of inhibiting skin damage caused by UV lights, and an excellent effect of reducing skin wrinkles.