Objective Formic, acetic, propionic and butyric acid exist widely in the natural ecological environment, but their contents are usually low. Thus, their accurate qualitative and quantitative determination is often hindered by impurities in the sample matrix.Method Using the coupling amidation method, low molecular monocarboxylic acids (LMMAs) can form new derivatives with high sensitivity to ultraviolet (UV) light. This was achieved by incorporating the indole group into LMMAs' structure, which made it possible for the low-level LMMAs to be detected by high-performance liquid chromatography with diode array detector (HPLC-DAD) under a specific wavelength UV light with high response. As a result, we observed the accurate determination of LMMAs. Through experimentations, we propose the following steps: first, a suitable amount of sample and MES (2-Morpholinoethanesulfonic Acid) aqueous solution were added into a small bottle with the reaction system adjusted to about pH≈5.5. This was followed by the addition of EDC (1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride) aqueous solution. After 20 minutes of cross-linking reaction, a suitable amount of NHS (N-Hydroxy succinimide) aqueous solution was added to promote coupling reactions. After an appropriate period, a tryptamine aqueous solution (containing acetonitrile) was added into the reaction system for amidation overnight. This was followed by the addition of ultrapure water to a certain volume and prepared for determination.Result The experimental results showed that for an optimized observation, the mole ratio of MES: EDC: NHS: tryptamine: carboxylic acid should be maintained at about 10 : 7 : 3 : 7 : 1 around and after 45 minutes of coupled reaction and the addition of tryptamine aqueous solution for amidation. The detection limit for the four organic acids in this method was from 0.008 to 0.046 mg·L^–1, and the linear determinate coefficient R² of the standard working curve was around 0.991~0.998. The average recovery rate range was 85.9%~123%, and the precision of the method ranged between 1.69%~8.41%, which meets the requirements of organic analysis.Conclusion The actual sample test results showed that the data of the derivation method and the direct method were the same. However, compared with the HPLC direct detection method, the derivation-HPLC method could correct the positive error caused by the impurities in complex samples, and improve the detection rates of low concentration monocarboxylic acid samples. So the qualitative and quantitative results of the derivation-HPLC method were more reliable.