Free-fall penetrometers (FFPs) are used to measure undrained shear strength of cohesive soils. FFPs are featured with quick operation and good economical efficiency, with significant potential in offshore in-situ investigations. However, the data interpretation of FFP tests is impaired by uncertainty in the dynamic penetration mechanism of FFPs. Reduced-scale model tests were conducted in kaolin clay with different strength profiles, whilst a simplified analytical model was modified to predict the undrained strength through the penetration velocity profile. The analytical model was validated by comparison with the testing data. Based on parametric study, the "FFP mass-impact velocity" contours are proposed for quick design of the in-situ tests given the rough range of undrained strength from local experiences.