The invention provides a square emulsion preparing device comprising a main stream inflow pipeline, a main stream contraction section, a main stream throat pipeline, a main stream diffusion section, a main stream outflow pipeline and a lateral face lead-in pipe, wherein the main stream inflow pipeline, the main stream contraction section, the main stream throat pipeline, the main stream diffusion section and the main stream outflow pipeline are sequentially communicated; the lateral face lead-in pipe is arranged at the upper end of the main stream throat pipeline and is communicated with the main stream throat pipeline; the main stream inflow pipeline, the main stream throat pipeline and the main stream outflow pipeline are square pipelines; the main stream throat pipeline and the main stream diffusion section are in a quadrangular-pyramid-like structure; channels I and channels II are respectively arranged on the upper lateral wall faces and the lower lateral wall faces of the main stream inflow pipeline, the main stream contraction section, the main stream throat pipeline, the main stream diffusion section and the main stream outflow pipeline; and the mounting positions of the upper wall face and the lower wall face of the emulsion preparing device can be adjusted according to demands in a quick assembly process. According to the square emulsion preparing device provided by the invention, the main stream inflow pipeline, the main stream outflow pipeline and the main stream throat pipeline are designed to be square pipelines, so that the convenience in machining can be realized, and a plurality of methyl alcohol solution lead-in pipelines are arranged on the lateral face of the throat part; and diesel flows through the main stream inflow pipeline, the main stream contraction section, the main stream throat pipeline, the main stream diffusion section and the main stream outflow pipeline, so as to form three-phase methyl alcohol, water and diesel emulsion.