A micro-bubble generator has an intake manifold, a casing threadingly connected to the intake manifold, a first air inlet channel defined between threads of the intake manifold and the casing, a booster located inside the casing and having a gap defined between the casing and the booster to form a second air inlet channel and to communicate with the first air inlet channel, a bubble generating tube located inside the casing and having a third air inlet channel defined between the end faces of the bubble generating tube and of the booster. The booster includes a first water inlet and a first water outlet having an inner diameter smaller than that of the first water inlet so that water velocity at the first water outlet is faster than that at the first water inlet, which forces ambient air to enter the bubble generating tube via air inlet channels and to be mixed with water in the bubble generating tube to generate bubbles. Bubbles are cut into micro-bubbles after passing through the cutter and exit the bubble exit.