Monitoring water sediment concentration has always been important in hydrological observation and water construction. Real-time and effective monitoring of water sediment concentration has important practical value. Traditional manual measurement methods are inefficient and unable to monitor in real-time. Although instrument monitoring methods based on an ultrasonic wave can realize the real-time measurement of sediment concentration in a water body, they have disadvantages in terms of safety, stability and measuring range. The technology of material content monitoring based on spectroscopy is fast, non-destructive, accurate and efficient and has been widely used in various fields in recent years, providing new ideas and methods for on-line monitoring of water sediment content. However, direct transmission spectroscopy is susceptible to the instability of the light source and the interference of stray light from outside, resulting in spectral noise. At the same time, due to the light intensity saturation of the instrument and equipment, its measuring range is limited. Based on this, this paper focuses on direct transmission spectrum noise processing and multi-section calibration technology and designs a fast and large-range on-line monitoring system for water-sediment content based on transmission spectrum. First, the relationship between sediment content and transmitted light intensity in a water body is analyzed based on Lambert-Bill’s basic law theory. Then, a sediment monitoring test system for a water body is established by using colorimetric dish holder laboratory, and standard solutions with different sediment content ratios are prepared to calibrate the actual correlation degree of intensity-sediment content. In order to overcome the influence of spectral noise, the original transmission spectrum is pre-processed by a wavelet threshold denoising algorithm. Spectral noise is eliminated using sym7 wavelet base, minimum and maximum threshold selection rule and 7 times of wavelet decomposition. With different integration times, a large range measurement of sediment content from 4% to 22% is realized by multi-section calibration, and the calibration function and measurement range are automatically matched in the algorithm. The results show that the R-square values of the calibration curves are all above 0.99, with good linearity and by the theory. Finally, the on-line monitoring system for water-sediment content is designed and tested for actual accuracy. The results show that the measurement errors are all controlled below 0.4% in the wide range of measurement, the mean of full range error is 0.173%, and the standard deviation of error is 0.115%, which can meet the actual requirements of the project. Therefore, an on-line monitoring system for water-sediment concentration over a wide range is proposed in this paper, and the system’s measurement accuracy is verified by tests, which can be used for real-time on-line monitoring of water sediment concentration.