The invention relates to an underground real-
time control system for the density of 
drilling fluid. The underground real-
time control system for the density of the 
drilling fluid comprises a drilling rig, an 
air compressor, a 
slurry pump, an intermediate transmission device, an 
electromagnetic valve, a packer, a 
wireless receiver and a ground 
control system, wherein the intermediate transmission device consists of an inner 
barrel and an outer 
barrel, which are coaxially arranged; the 
air compressor is connected to an air inlet of the inner 
barrel; the 
slurry pump is connected to a fluid inlet of the outer barrel; the 
tail end of the inner barrel is connected with an inner 
drill rod; the 
tail end of the outer barrel is connected with an outer 
drill rod; the 
tail end of the outer 
drill rod is connected with a drill collar; the tail end of the inner drill rod is fixedly arranged at the bottom of the drill collar; the tail end of the inner drill rod is connected with the 
electromagnetic valve; the packer is arranged on the outer drill rod; a 
wireless density sensor and a 
wireless pressure sensor are arranged at the bottom of the drill collar; the wireless 
receiver is connected with the ground 
control system. According to the 
system, the advantages of three drilling technologies, i.e. an overbalance drilling technology, an under-balanced drilling technology and a managed pressure drilling technology, are fully utilized, so that an underground 
pressure difference can be switched between an overbalanced state, an under-balanced state and a near-balanced state, the drilling rig can safely and efficiently reach a destination layer, the non-production time can be reduced, and the drilling cost can be lowered.