Using direct ultraviolet-visible (UV-visible) spectroscopy method to detect water quality parameters has some advantages such as rapid detection speed, low cost, no secondary pollution, on-line and in-situ measurements. However, the detection system based on this method has been difficult to promote be cause of the limitation of to the accuracy of spectral detection and the cost of equipment. In this paper, we use the industrial-grade USB2000+ spectrometer and pulsed xenon light source of Ocean Optics to build a low- cost, rapid water quality spectral detection system. Furthermore, on the basis of a comprehensive analysis of the system noise sources, we use the soft thresholding wavelet packet method to denoise the raw spectral data. In order to effectively evaluate the denoising effect, we use scientific-grade spectrometers QE65000 of Ocean Optics with stable deuterium tungsten light source to build a standard spectral detection system, and carry out UV-visible absorption spectrum acquisition on the same sample solution. Experimental results show that the proposed system can capture the raw spectral data stabl and effectively. Then using soft thresholding wavelet packet to denoise the original data, on the premise of retaining water samples UV-visible absorption spectrum characteristics, various noises of the original spectra are effectively removed and the signal-to-noise ratio, which makes a foundation for the low-cost spectral detection device for the analysis of water quality parameters.