The invention provides an underwater releasing device with two trigger modes of pure mechanical timing and pure mechanical depth setting. The underwater releasing device comprises a mechanical timing starting valve, a depth setting blasting mechanism and a releasing mechanism, wherein the mechanical timing starting valve consists of a timing mechanism, a linkage mechanism and a valve, and radial sealing is realized by a mechanical timing rotary button and a valve energy storing rotary button with an upper end cover through an O-shaped sealing ring; a timer shaft and a ball valve shaft are in the coaxially hinged connection design with a rotary button shaft, the timer shaft and the ball valve shaft are provided with gear rods, and the rotary button shaft is provided with a shifting rod; a timer cam is in contact with a linkage sliding shovel, a linkage lock tongue is in contact with a valve clamping disk, and energy is stored by the ball valve shaft through torsional springs; the opening of a ball valve is connected with an accommodating cavity of the releasing mechanism; a hole is formed in an upper cover of the depth setting blasting mechanism and communicated with outside, grease exists in an oil sac, and a blasting piece pressure relief opening is communicated with the accommodating cavity of the releasing mechanism; a steel ball is blocked by the releasing mechanism, so that the steel ball is prevented from rolling inwards, and an inner cylinder body is prevented from falling downwards. The underwater releasing device disclosed by the invention has the two trigger modes and can be used for underwater robots, underwater engineering machinery, underwater engineering equipment and the like.