For the first time this book proposes a 3D rigid human model to do a mechanical analysis for Taijiquan. The model comprises the common mechanical conditions, including the frame, mass, velocity, attacking force, gravity, normal force, friction force, etc. The model is used to analyze Taijiquan, which results in a few important conclusions. The gravity center should be inside the supporting area to maintain the balance. Skidding and root loss are two types critical point that lead to loss of control. The body is in balance within the critical points. Beyond one of the critical point, the body will eventually loose balance. The attacking force comes from the friction force, normal force, the body overall momentum, and the momentum of the relative motion between limbs. The interior strength creates body momentum and transfer force. The critical points are calculated for taiji solo and push hand. Numerical examples demonstrate the Taiji skills of "throw like an arrow," "induce into empty," "four ounces deflect thousand pound" by mechanical principle. Taijiquan belong to sports, is human limb movements and body reaction. Taijiquan is a martial art, is mental activity and fighting mechanism. Taijiquan complies with mechanical principle, is the interaction among opponents and ground. Analyzing Taijiquan techniques by contemporary mechanical laws clarifies the mystery of the ancient theories, modernize the method to explore Taijiquan, promotes scientific development and international dissemination for Taijiquan, and produces great practical consequences and far-reaching significance.