The invention discloses a device for measuring an effective atomic number of a substance, which comprises an X ray source which emits X rays with different energy to a measured object, a Cerenkov detector which receives the X rays passing through the measured object and generates a first electrical signal, an energy depositing detector which receives the X rays passing through the measured object and generates a second electrical signal, and a processor which is used for acquiring the effective atomic number of the measured object according to the first electrical signal and the second electrical signal. The invention also discloses a method for measuring the effective atomic number of the substance. The method for measuring the effective atomic number of the substance by combining the Cerenkov detector and the energy depositing detector ensures that X-ray photons with high energy are more easy to form large signals during the detection of the Cerenkov detector compared with the X-ray photons with low energy, so the resolution effect of the acquired effective atomic number of the substance is enhanced.