Abstract: A low-cost confocal fluorescence detector was developed using 3D printing technology and applied to the determination of ammonium nitrogen in river water samples. The detector used a confocal structure, and utilized light-emitting diode (LED) with a wavelength of 430 nm as light source, along with optical components such as a ball lens, plano-convex lens forming a collimating lens and filters. Various modules were produced by 3D printing technology for fixing various optical components. Quinoline-2, 3-dicarbaldehyde (QDA) was synthesized as the fluorescent reagent for ammonium nitrogen detection. QDA reacts with NH₄⁺-SO₃²⁻-Ca²⁺, and the product has a maximum excitation wavelength of 429 nm and maximum emission wavelength of 518 nm. The developed detector, along with the synthesized QDA reagent were used to establish a fluorescence detection method for ammonium nitrogen. The limit of detection of the method was 0.038 μmol/L (3σ), and the relative standard deviation (RSD) was 2.21% for detecting 3.0 μmol/L ammonium nitrogen standard solution. The spiked recoveries ranged from 89.8% to 93.7% when applied to the detection of river water samples. Leveraging 3D printing technology facilitated the development of the detector, the manufacturing costs were reduced and the rapid optimization of the structure was enabled. The developed detector demonstrates excellent stability, high sensitivity, and a simple structure, making it suitable for rapid sample testing in environmental and food analysis applications.