The utility model discloses an underwater tube-type slug flow buffer which comprises a mixed-phase inlet, a liquid phase distribution pipe and a gas phase distribution pipe, wherein one end of the liquid phase distribution pipe is communicated with the mixed-phase inlet, and the other end of the liquid phase distribution pipe is a gas-rich liquid phase outlet; the liquid phase distribution pipe is communicated with the gas phase distribution pipe by a plurality of gas-liquid communicating pipes; one end of the gas phase distribution pipe is a liquid-rich gas phase outlet. After the underwater tube-type slug flow buffer is used, the flow pattern of slug flow can be distributed again, so that the impact of liquid phase to a separator can be reduced. The slug flow buffer is designed into tube type, so that the manufacturing difficulty and cost of the separator are greatly reduced, and the underwater installation difficulty is also reduced; after the underwater tube-type slug flow buffer is arranged at an entrance of the traditional or any gas-liquid separator, the separation efficiency of the separator can be greatly improved.