Objective Comparison of two methods for determining ammonium ion content in ambient air${\mathrm{{PM}}}_{2.5}$. Methods $1/4$ of the quartz filter membrane used to collect ambient air particles${\mathrm{{PM}}}_{2.5}$ was cut into pieces, and ultrasound extracted by ultra-pure water, then centrifuged and filtered, and last the content of ammonium ion in${\mathrm{{PM}}}_{2.5}$ was respectively determined by flow injection method and ion chromatography. Result The fitting curve of${\mathrm{{NH}}}_{4}^{+ }$ measured by ion chromatography in the range of${0.5}\sim {10.0}\mathrm{{mg}}/\mathrm{L}$ is$\mathrm{y}= {0.0710}+ {0.2214}\mathrm{x}- {0.0058}{\mathrm{x}}^{2},\mathrm{r}= {0.9997}$ . The detection limit of${\mathrm{{NH}}}_{4}^{+ }$ in${\mathrm{{PM}}}_{2.5}$ is calculated to be${0.017}\mathrm{{\mu g}}/{\mathrm{m}}^{3}$ , with a quantification limit of${0.068\mu }\mathrm{g}/{\mathrm{m}}^{3}$ , the recovery rate is between${91.0}\%\sim {96.6}\%$ , and the RSD is${0.05}\%\sim {0.08}\%\left({\mathrm{n}= 6}\right)$ . The linear relationship of ammonium ion concentration measured by flow injection analysis in the range of${0.02}\sim {1.00}\mathrm{{mg}}/\mathrm{L}$ is good, with$\mathrm{y}= {22.576}\mathrm{x}+ {0.1333},\mathrm{r}= {0.9999}$ . The detection limit is${0.003}\mathrm{{\mu g}}/{\mathrm{m}}^{3}$ , and the quantification limit is${0.012\mu }\mathrm{g}/{\mathrm{m}}^{3}$ with the recovery rate of${105}\%\sim {114}\%$ and the RSD between${0.54}\%\sim {2.06}\%\left({\mathrm{n}= 6}\right)$ . Using the two methods to measure 16 filter membrane samples, there was no statistically significant difference in the measurement results$\left({\mathrm{t}= {1.080},\mathrm{P}> {0.05}}\right)$. Conclusion Both of the two methods have a good linear relationship, with good precision and accuracy. Compared with ion chromatography, flow injection analysis has the advantages of lower detection limit and shorter detection time.