Real-time online detection of fruit quality system has been applied to production practice because online testing and grading of fruits screening technology has matured. However, fruit size and quality online testing have always been difficult. Many detection methods of fruit size and quality are very complicated and time consuming, which cannot meet the needs of real-time detection. In this paper, a new method for rapid detecting small watermelon of volume and quality was based on the X-ray image processing. The volume and quality of the relevant model established the predicting outcome of coefficient of determination R2 obtained were 0.9858 and 0.9922 respectively, showing that these two models have high correlation and predicting the volume and quality of watermelon accurately as well as laying the foundation for the watermelon online detection and classification.