The present invention discloses a system for measuring the mechanical properties of sea floor sediments at full ocean depth. The system includes an overwater monitoring unit and an underwater measurement device, where the underwater measurement device includes an observation platform and a measuring mechanism; the observation platform includes a frame-type body and a floating body, a wing panel, a floating ball cabin, a leveling mechanism, a counterweight, and a release mechanism mounted on the frame-type body; the floating ball cabin seals a circuit system; the leveling mechanism adjusts the underwater measurement device horizontally on the sea floor when the frame-type body reaches the sea floor; the release mechanism discards the counterweight for recovery of the unit after the underwater measurement device completes the underwater operation; the measuring mechanism includes at least one of a cone penetration measuring mechanism, a spherical penetration measuring mechanism, and a vane shear measuring mechanism, or a sampling mechanism.