A cable splice box, characterized by comprising: an end surface passing therethrough a cable having two cable segments respectively passing in and out the cable splice box; a first waterproof structure, including: a pillar including: a pillar body having at least two longitudinal grooves, each of which has a bore diameter X configured to accommodate one of the two cable segments; a longitudinal surface having at least two longitudinal slits, each of which has an operating slit width Y larger than or equal to X, corresponds to one of the at least two longitudinal grooves, and forms an opening thereon; and at least two longitudinal elastic pieces, each of which forms one edge of one of the at least two longitudinal slits; and a hollow cylindrical tube disposed on the end surface, having an outer surface and providing a mid-span branch connection; and a second waterproof structure including an elastic contracting sleeve disposed on the outer surface and enveloping the first waterproof structure, characterized in that the elastic contracting sleeve exerts a radial contracting force on the first waterproof structure to continuously maintain tightness between the pillar body and the two cable segments included in the first waterproof structure, and the at least two longitudinal elastic pieces are in place both before and after formation of the first and second waterproof structures (12, 16). an end surface passing therethrough a cable having two cable segments respectively passing in and out the cable splice box; a first waterproof structure, including: a pillar including: a pillar body having at least two longitudinal grooves, each of which has a bore diameter X configured to accommodate one of the two cable segments; a longitudinal surface having at least two longitudinal slits, each of which has an operating slit width Y larger than or equal to X, corresponds to one of the at least two longitudinal grooves, and forms an opening thereon; and at least two longitudinal elastic pieces, each of which forms one edge of one of the at least two longitudinal slits; and a hollow cylindrical tube disposed on the end surface, having an outer surface and providing a mid-span branch connection; and a pillar including: a pillar body having at least two longitudinal grooves, each of which has a bore diameter X configured to accommodate one of the two cable segments; a longitudinal surface having at least two longitudinal slits, each of which has an operating slit width Y larger than or equal to X, corresponds to one of the at least two longitudinal grooves, and forms an opening thereon; and at least two longitudinal elastic pieces, each of which forms one edge of one of the at least two longitudinal slits; and a pillar body having at least two longitudinal grooves, each of which has a bore diameter X configured to accommodate one of the two cable segments; a longitudinal surface having at least two longitudinal slits, each of which has an operating slit width Y larger than or equal to X, corresponds to one of the at least two longitudinal grooves, and forms an opening thereon; and at least two longitudinal elastic pieces, each of which forms one edge of one of the at least two longitudinal slits; and a hollow cylindrical tube disposed on the end surface, having an outer surface and providing a mid-span branch connection; and a second waterproof structure including an elastic contracting sleeve disposed on the outer surface and enveloping the first waterproof structure, characterized in that the elastic contracting sleeve exerts a radial contracting force on the first waterproof structure to continuously maintain tightness between the pillar body and the two cable segments included in the first waterproof structure, and the at least two longitudinal elastic pieces are in place both before and after formation of the first and second waterproof structures (12, 16).