The invention provides a preprocessing system for near-infrared online detection, which is composed of a filter, a back-up filter, a variable-frequency centrifugal pump, a temperature control device, a buffering device, a flowmeter, a near-infrared spectrum acquisition device, an automatic sampling device, a liquid inlet and a liquid outlet. The preprocessing system provided by the invention can run automatically, continuously and reliably, and can be switched for use under the condition of not affecting the function of the whole system, therefore, the automatic positive and negative washing can be realized simultaneously, the samples of acquired spectrums can be acquired in a targeted mode, and the cleaning and beauty of the whole system can be maintained. By using the preprocessing system provided by the invention, the problem of difficulty in detection caused by more insoluble solid particles, unstable flow, easily-produced bubbles and large temperature fluctuation in the process of near-infrared online detection is solved, so that the acquired near-infrared spectrum is stable, reliable, and good in repeatability, thereby improving the accuracy of results of near-infrared on-line detection, promoting the development and application of the near-infrared spectral analysis technology in pharmaceutical production processes, and then truly realizing on-line quality control.