The invention relates to a dual-density well drilling underwater hollow ball injection device. The dual-density well drilling underwater hollow ball injection device comprises a hollow ball injection connector, a diluent injection branch pipeline, a first-stage separator, a second-stage separator connection pipeline, an anchor, a carrier liquid discharging opening, a second-stage separator, a hollow ball discharging pipeline and a diluent injection pipeline connector. The separators are fixed to the anchor and are put into the seabed through a down-pass device, and the anchor is inserted into soil to fix the separators; diluent enters the first-stage separator for first time of separation through the diluent injection pipeline connector and the diluent injection branch pipeline, but is not completely separated; and a base solution of the first-stage separator enters the second-stage separator through the second-stage separator connection pipeline for separating hollow balls again, the hollow balls separated out twice all enter a riser annulus through the hollow ball discharging pipeline, the hollow ball injection connector and a hose, and a second-stage separation base solution is directly discharged. The high-purity hollow balls separated by the separators enter the riser annulus to dilute mud, so that dual-density well drilling is implemented successfully.