Abstract: A spectrophotometric method of determining the color reaction of thorium(Ⅳ)with piperonyl fluorone is studied by an atomic absorption spectrophotometer.The maximum absorption of this system is at 560nm with molar absorptivity of 1.12+×10⁵ L·mol^-1·cm^-1.Beer′s law is obeyed in the range of 0~25μg(25mL)^-1.The method has been applied to the determination of thorium(Ⅳ)in geochemical standard reference samples.The results obtained consistent with the certified values.