Abstract: Navier- Stokes equation of incompressible fluid is derived according to the idea and method in classical Newtonian mechanics. This method which combines the cross-section method in mechanics of elasticity， the concept of fluid micelle and the fluid continuity equation and so on. It is an exercise for students' thinking 　　ability，while it improves the ability of comprehensive utilization of knowledge.

Key words: Newton's law, Navier-Stokes equations, fluid