The invention belongs to the technical field of municipal engineering and environmental protection and in particular relates to an initial rainwater flow distributing device in a rainwater inspection well, targeting at interception of initial rainwater. The initial rainwater flow distributing device comprises an automatic turnover door system arranged nearby a pipe orifice at the downstream of the rainwater inspection well, a small connecting pipe between rainwater and sewage pipelines and a check valve arranged at the tail end of the small connecting pipe. At the initial stage of rainfall, high-concentration initial rainwater enters a sewage pipe network via the small connecting pipe through an orifice between a balance turnover plate and the bottom of a rainwater pipe. When flow of the sewage pipeline reaches full pipe flow, the check valve at the tail end of the small connecting pipe is pushed to be closed, and rainwater is prevented from continuously entering the sewage pipeline. Along with sustained rainfall and further water level rise in the pipeline, the automatic turnover door system automatically turns over for 90 degrees along the flow direction, the small connecting pipe between the rainwater and sewage pipelines is closed, and rainwater is discharged to the downstream rainwater pipeline. After rainfall ends, the water level in a water inlet pipe is lowered, a balance baffle automatically turns over to an original position, and the whole turnover door system is driven to return to the working condition before the rainfall.