The invention provides a high-efficiency washing and decontamination device for a methane project by a pressure water washing method, and relates to the technical field of methane purification and decontamination equipment. The high-efficiency washing and decontamination device comprises a tower body, wherein the tower body comprises at least two packing sections; a water storage section is arranged at the bottom of the packing section at the bottommost end; a liquid outlet is formed at the bottom of the water storage section; a gas inlet is formed at the upper part of the water storage section; a liquid inlet is formed at the top of each packing section; a gas outlet is formed at the top of the tower body; diameters of the packing sections are sequentially reduced from the bottom up; a random packing layer and a structured packing layer are arranged in each packing section; and material distributors are arranged above the structured packing layers. According to the high-efficiency washing and decontamination device, the tower body consists of the packing sections with the different diameters, so that the diameter of the tower body is reduced; one-time investment of the device is saved; circulation consumption of water is reduced; energy consumption is reduced; phenomena such as a wall flow and non-uniform distribution of gas and liquid phases in the packing layers caused by a wall flow effect are avoided; and the absorption efficiency of the device is improved.