The invention discloses a high-strength cable sealing sheath for underwater detection and a preparation method thereof. By controlling process steps and raw material composition of the high-strength cable sealing sheath for underwater detection, the high-strength cable sealing sheath for underwater detection is prepared through processes of two-time internal mixing, one-time open mixing and one-time extrusion. Ingredients of the cable sealing sheath are subjected to two-time internal mixing, one-time open mixing and one-time extrusion, and the strength and corrosion resistance of the prepared cable sealing sheath are remarkably improved especially under the condition that the temperature of two-time internal mixing is 17-37 DEG C higher than that of one-time internal mixing. Not only that, the cable sealing sheath is simple in preparation method and easy to popularize and has high application value.