Based on the analysis of industry helical motion, spiral motion model is established, and a "total error" concept is proposed for its error analysis.

Under the precondition of incompressible flow, fluid spiral motion in SK static mixer tube was resolved into axial beeline motion and circumferential rotation motion.

A small ball is making circular motion at the uniform velocity, the centripetal force has suddenly increased, it is proved that the small ball will generally make a spiral motion.