The invention provides a small-section rotary spraying drilling tool for porous pipes. The outer diameter of the small-section rotary spraying drilling tool for the porous pipes is smaller than 100 mm. Preferably, the outer diameter of the small-section rotary spraying drilling tool for the porous pipes is 90 mm. The small-section rotary spraying drilling tool for the porous pipes comprises a rotary spraying drill bit; the rotary spraying drill bit comprises a rotary spraying drill bit body and a gas-slurry coaxial nozzle; the gas-slurry coaxial nozzle is arranged on the side surface of the rotary spraying drill bit body; the axis of the gas-slurry coaxial nozzle is obliquely arranged with the axis of the rotary spraying drill bit body; the rotary spraying drill bit body is provided with a first end part and a second end part; the gas-slurry coaxial nozzle is inclined towards the second end part; and preferably, the axis of the gas-slurry coaxial nozzle and the axis of the rotary spraying drill bit body are arranged to form an included angle of 70 degrees. The small-section rotary spraying drilling tool for the porous pipes can be used for reinforcing a foundation in a subway tunnel bypass channel construction ground by high-pressure jet grouting with controllable pressure and no mud discharge, can reinforce a soil body around the outer surface of a tunnel by high-pressure jet grouting, and is ingenious in design, simple in structure, simple and convenient to manufacture and suitable for large-scale popularization and application.