The initial runoff diversion device is designed to automatically discharge the heavily polluted rainwater from the early stage of rainfall while providing preliminary filtration for the subsequent collected rainwater. During the initial phase, a small amount of rainwater directly impacts the stainless steel ball that controls the discharge pipe. Before the ball closes, this initial rainwater is discharged directly through the pipe under gravity. As rainfall intensifies, the stainless steel ball automatically closes, redirecting the rainwater to flow through a filter screen and into the collection pipeline, thereby completing the processes of initial runoff diversion and filtration.