Real-time detection of sugar content plays a crucial role in the estimation of Hetao melon quality. To reduce the effect of size on the diffuse transmittance spectra, an on-line detection testing system based on diffuse transmittance spectra and image information was designed for the evaluation of sugar content of melon. Two main parts (hardware and software) were involved in this device. The hardware includes delivery device, spectrum detection device, image capture device and control system. The software system was designed based on Microsoft Visual C ++ 6.0 language and combined with the software of Omni Driver, FlyCapture2 and OpenCV. With the proposed system, firstly, the spectrum and images of melon were automatically obtained, displayed and saved. Secondly, the sensitive spectrum, color (R, G, B) and volume value of melon were derived by processing spectra and image information. Finally, the sugar content of melon was obtained according to the model of diffuse transmission spectrum and image information fusion. In conclusion, this device could automatically calculate the amount of melon, timely display and save the color value, volume value and sugar content of melon in computer, and detect a sample within 1.2s with root mean square errors of 1.22 for sugar content. The proposed system was showed to be a promising tool for implementing on-line detection of Hetao melon sugar content.