The use of immobilization methods to treat radioactive nuclear waste liquid is one of the most effective ways for preventing radioactive contamination. Understanding the solidification matrix for the immobilization behavior of radioactive ions is an important issue. In this work, ab initio calculation was used to study the solidification mechanism of Cs+ in K-struvite crystal in the aqueous solution. Corresponding experiments were carried out and analyzed by XRF11X-ray fluorescence spectrum, XPS22X-ray photoelectron spectroscopy, AAS33Atomic absorption spectrometry, FI-IR44Fourier transform infrared spectrometer, SEM55Scanning electron microscope, EDS66Energy dispersive spectroscopy and XRD77X-ray diffraction . This work proposed a calculation method for the ΔEaq88Aqueous Solution Substitution Forming Energy is abbreviated as ΔEaq. . By analyzing ΔEaq, the tendency of Cs+ to occupy the K site in K-struvite crystal is remarkable. Furthermore, in-depth first-principles calculations show that this immobilization behavior is related to the electronic structures of K-struvite and Cs-struvite99For convenience, [K, Cs]-struvite is abbreviated as Cs-struvite. K, Cs each accounted for 50%., and they together with the interaction of cation and aqueous solution determine the energy change of the process. This work provides a basic perspective for the study of magnesium phosphate potassium cement solidified nuclides, which is convenient for judging the solidification of other nuclide ions, thereby designing K-struvite cement for solidified nuclide. This method is also readily extended to other studies of the chemical immobilization of any crystal in the solution to any other ions.