An optical fiber cable splice enclosure for mid-span splicing operation of a backbone cable , characterized by comprising the following: a cable entry board ; a connection part configured on the cable entry board , wherein the backbone cable has a first segment extending through the connection part to form a U-turn in the optical fiber cable splice enclosure and second segments extending through the connection part out of the optical fiber cable splice enclosure ; a first hollow cylindrical tube disposed on the connection part ; and a flexible shrinkable tube having one end waterproofing a connection between the first hollow cylindrical tube and the connection part , and a second end waterproofing the first and the second segments at an external side of the cable splice enclosure ; and a gap-filling waterproofing auxiliary device comprising two pieces of semi-elliptical constrictive enclosure with dual concave sinks for containing the second segments of the backbone cable , wherein the two pieces of semi-elliptical constrictive enclosure are screwed together to form an elliptical waterproof structure, wherein a double-sided adhesive waterproof tape is arranged between the two pieces of semi-elliptical constrictive enclosure and between each second segment of the backbone cable and the dual concave sinks, wherein an elastic waterproof pad with a single adhesive side wraps the outer side of the semi-elliptical constrictive outer enclosures , wherein the flexible shrinkable tube wraps the outer part of the first hollow cylindrical tube and a portion of the elliptical waterproof structure, which does not penetrate the first hollow cylindrical tube . a cable entry board ; a connection part configured on the cable entry board , wherein the backbone cable has a first segment extending through the connection part to form a U-turn in the optical fiber cable splice enclosure and second segments extending through the connection part out of the optical fiber cable splice enclosure ; a first hollow cylindrical tube disposed on the connection part ; and a flexible shrinkable tube having one end waterproofing a connection between the first hollow cylindrical tube and the connection part , and a second end waterproofing the first and the second segments at an external side of the cable splice enclosure ; and a gap-filling waterproofing auxiliary device comprising two pieces of semi-elliptical constrictive enclosure with dual concave sinks for containing the second segments of the backbone cable , wherein the two pieces of semi-elliptical constrictive enclosure are screwed together to form an elliptical waterproof structure, wherein a double-sided adhesive waterproof tape is arranged between the two pieces of semi-elliptical constrictive enclosure and between each second segment of the backbone cable and the dual concave sinks, wherein an elastic waterproof pad with a single adhesive side wraps the outer side of the semi-elliptical constrictive outer enclosures , wherein the flexible shrinkable tube wraps the outer part of the first hollow cylindrical tube and a portion of the elliptical waterproof structure, which does not penetrate the first hollow cylindrical tube . <EOS>