The self-rotating kinetic moment formula is derived through theoretical dynamic research on self-cleaning and rotating steel spiral with plastic coat. The moment is proportional to d (spiral wire diameter), sin2α (α-helical angle), D2 (D-spiral O.D), u2 (u-flow velocity), ρ (fluid density). The theoretical formula is consistent with experimental results. As a result, the formula can be the theoretical foundation to optimally design the steel spiral with plastic coat. The spiral is thicker with coat but the experiment results show the flow resistance is still in the general range of industry limitation.