Abstract: To improve the thermostability of the phytase YiAPPA by introducing ionic bonds through site-directed mutagenesis. By comparing the molecular structure of YiAPPA and phytase rPhyXT52 with excellent thermal stability, site-directed mutagenesis was used to introduce molecular surface ionic bonds related to the thermostability of rPhyXT52 into YiAPPA to construct mutants. The mutant T209K/S220E/N237D with significantly improved thermo stability were obtained through screening. The mutant T209K/S220E/N237D had an specific activity of 3982.06 U/mg at 37 °C and pH4.5, which was essentially consistent with the respective values for YiAPPA. The optimal reaction temperature, optimal reaction pH, pH stability, and protease resistance of T209K/S220E/N237D were also basically the same as those of YiAPPA. However, compared with that of YiAPPA, the half-life of T209K/S220E/N237D at 80 °C was extended from 14.81 to 24.72 min, the half-inactivation temperature T₅₀³⁰ was increased from 55.12 to 64.05 °C, and the T_m value was increased from 48.36 ℃ to 55.04 ℃. Molecular dynamics simulations showed that the introduction of new ionic bonds in T209K/S220E/N237D improved the stability of the structural units in the enzyme molecule in which the amino acid residues constituting these ionic bonds were located, thereby improving the thermostability of the enzyme. Results showed that the introduction of ionic bonds into YiAPPA could effectively improve its thermostability and make it more applicable for food processing. The results would be also provide a theoretical basis for the improved thermostability of phytase and other enzymes.