‘Hawk-Eye’ system started to apply into tennis competition as early as 2003, now almost all large-scale tennis competitions use ‘Hawk-Eye’ system to define tennis drop point so as to ensure competition accuracy and fairness. By kinematics knowledge, mathematical differential, adopt several shapes combination, and with the help of MATLAB software, it researches and discusses ‘Hawk-Eye’ system working principle and accuracy. In research process, it discusses by dividing tennis into taking spin force into consideration and ignoring spin force such two cases. Firstly, it converts tennis three -dimensional space coordinate into motion plane’s two dimensional coordinate, and then utilizes kinematics knowledge, it gets tennis kinematical equation, uses mathematical differential and Bernoulli equation to simplify it and then can get tennis drop point. To judge the system accuracy, select one competition tennis positions records, input them into model, and get that tennis drop point is nearly the same as tennis actual drop point in competition, it is clear the system accuracy is high.