The invention belongs to the technical field of submersible ultra-deep protection and relates to a submersible ultra-deep protective device. The submersible ultra-deep protective device comprises an electronic protection unit, a mechanical protection unit and a floating load throwing unit. A protection method includes the steps that an MCU (Microprogrammed Control Unit) of the electronic protection unit determines whether a submersible works at the safety depth or not according to the working depth and the height to the bottom of the submersible, which are obtained through a depth meter and a height meter, and the MCU controls the floating load throwing unit to throw away a floating load if the working depth of the submersible is too large or the height to the bottom of the submersible is too small; a rubber pad of the mechanical protection enables a reed to perform elastic deformation to push a push rod to move through the pressure in deep water, and when the working depth is too large, a micro switch is touched to act and a normally closed contact is disconnected to throw away the floating load. The electronic protection unit and the mechanical protection unit are connected in series and the floating load throwing unit can be triggered to throw away the floating load when either the electronic protection unit or the mechanical protection unit is activated, so that the safety of the submersible is protected maximally, wherein the safe working depth/height of the submersible can be set according to the actual situation. The submersible ultra-deep protective device has high applicability.