Infant's fall accidents are occurring frequently. For comprehensive quantitative estimation of injury risks, it is important to divide the injured time into evasive action, protective reaction, and passive deformation. However, the protective reaction of kinematic and kinetic properties has not been clear. Therefore, in order to solve the mechanism of protective reaction, we construct a biomechanical analysis method of protective reaction. First, we obtained the joint angle data of the actual falling behavior of infants, and investigated the time of protective reaction with the cluster analysis. In kinematic analysis, we investigated the feature of falling motion using principal component analysis. As a result, the shoulder and knee extension movement was represented in second principal component. In kinetic analysis, we identified the joint torque with the fall simulation to reproduce the behavior of protective reaction. As a result, the protective reaction consisted of extension or flexion of limbs and restraint of the body trunk.