Abstract: To study a nondestructive inspection method of watermelon ripeness, under the measuring of the basic physical and mechanical properties of skin, pulp, and density of watermelon, a mechanical model was developed and impulse response experiments were carried out. The results showed that there was a better correlation between the first natural freqency(f) and sugar contents of watermelon. Furthermore, mass(m) of watermelon was taken into account, and ripeness index (E_d) was proposed as f²m^2/3. The relationship between E_d and sugar content was studied, and the E_d scope for “Jingxin” watermelon was given preliminary. The result showed that the impulse response is a potential online method for nondestructive inspection of watermelon.