Under-balanced drilling with nitrogen-foam is an international mature technology with advantages of low pressure of hydrostatic, less leakage, strong cutting carrying capacity, less damage to oil-gas layer and safe drilling in water-sensitive layer. With this technology, low pressure and low permeability oil-gas reservoir can be discovered and protected furthest for much higher economic benefit than conventional over-pressure well. It was applied in DF1 Well of Danudi gas field of low pressure and water-sensitive layer, and pollution on producing formation by ordinary slurry was solved.