The invention provides a method for preparing alternative fuel for a cement kiln by using composite material waste. A glass reinforced plastic composite material waste is crushed into grains with a grain diameter less than 20mm or slices to be used as the alternative fuel; the alternative fuel is sent into a preburning furnace of the cement kiln and is burnt under a pressure of -500Pa to -800Pa; or the alternative fuel is sprayed into the cement kiln from a multichannel burner at the head of the cement kiln. Through the method provided by the invention, the difficulty in treatment of the glass reinforced plastic composite material waste can be overcome and the fuel problem caused by high energy consumption of the cement industry can be solved.