Disclosed is an underground coal preparation process adopting a water medium, comprising: feeding raw coal into a special underground compact jigging machine to discharge overflow clean coal being dewatered through a φ1 mm fixed screen and screened through a φ13 mm classifying screen, and dewatering powder clean coal with a particle size less than 13 mm to obtain powder clean coal and a centrifugate; grading jigging middling coal by means of a φ13 mm classifying screen into block middling coal and powder middling coal, and dewatering the powder middling coal to obtain powder middling coal and a centrifugal liquid; and sending produced coal slime water to a water medium classifying and separation cyclone, and the obtained coarse middling coal slime and coarse clean coal slime respectively passing through a coarse middling and clean coal slime sieve bends, and then being dewatered.