Primary bremsstrahlung spectrum of X-ray tube has serious influence for trace Cadmium analysis in traditional EDXRF. Secondary targets with different geometry sizes were studied by Geant4 code. To enhance the efficiency of Geant4 simulations, the simulation processes were divided into three stages. In the first stage, primary spectra at different tube voltages were acquired using Geant4 code to simulate electrons of different voltages hitting anode target. In the second stage, Te and BaSO₄ of different kinds and geometry as secondary target materials, simulated. The simulation results show that when Te, whose K_α1 energy (27.468 keV) closes to the absorption limit of Cd (26.711 keV) is used as the fluorescence target material, the characteristic peak intensity of Te increases rapidly before 100 μm with the increase of target thickness, and tends to be stable after 150 μm. However, the signal to noise ratio (SNR) reaches the maximum value of 21.434 at 80 μm. Due to the self-absorption effect of the secondary target material, SNR declines slightly and becomes stable after reaching the saturation absorption thickness. In different application scenarios, the materials of the secondary target should be various. When there is no limit to the measurement time, the secondary target with greater fluorescence intensity should be selected. But, when the measurement time is relatively short, the secondary target of greater SNR should be selected. In the third stage, output spectra of the secondary target were used to activate sample containing 0.01% cadmium element. The output spectra of the Te element secondary target were used to activate samples, and the peak-to-background ratio of the K_α1 peak of Cd element was 8.28. The primary spectra were used to activate samples, and the peak-to-background ratio is 2.29. Although it has a great increase, the scattering peak of the Te element always influences the K_α1 peak of the Cd element. The BaSO₄ was selected as secondary target material because the characteristic X-ray energy is farther away from the K_α1 peak of the Cd element. The decrease of the peak-to-background ratio of target element could be weakened caused by the matrix elements of the sample. The peak-to-background ratio is increased to 14.179. The activation effect can be further improved by increasing the tube voltage of the X-ray tube. The optimal peak-to-background ratio of 21.431 could be obtained at the 70 kV tube voltage.