A free fall ball penetrometer with a booster is dynamically penetrated into the seabed through its kinetic and potential energies. The main measuring instrument is a ball penetrometer, which is subject to end bearing resistance, drag force, and soil buoyant force during the dynamic penetration process within the soil. Based on the measured data from the accelerometer and load cell, the soil strength parameters including the undrained shear strength and strain-rate parameter can be back-analyzed. The added booster can: (1) effectively increase the penetration depth of the ball penetrometer and hence enlarge the range of measured penetration depths; and (2) improve the directional stability and avoid the rotation of the ball penetrometer during the falling process. The force data measured from the load cell, together with the acceleration data from the accelerometer, can further improve the measured accuracy.