Abstract: In the teaching process of mechanics, the projection problem of vector equation is often encountered. If both sides of vector equations are equal, then the both sides of their projections will be equal whether it is one-dimensional, two-dimensional or three-dimensional problems. Taking the teaching of theoretical mechanics as an example, the application of the projection principle of vector equality is illustrated by the examples of statics, kinematics and dynamics respectively. The projection formula for each vector equation of each different chapter is avoided as a description or proof. Vector equality projection principle is also available to other courses in mechanics, college physics and electrotechnology which include vector equations.