A method using only one projector and one camera is proposed to build a Projector-Camera-Touchpad (PCT). This HCI system uses projector-camera to catch the finger’s motion to make any plane to a touchpad. This paper analyzes the error measuring the 3-dimentional coordinates of the system, and improves the accuracy by optimizing the calibration. To solve the problem of height when to touch happens and finger’s position accuracy, based on a finger model the color structured light is adopted to get 3-dimension information and locate the finger touch point. At meanwhile, the trajectory of finger touch is optimized by Kalman filter. Finally, the touch accuracy is improved to 99% under 2mm and touch position precision are increased to 1 pixel. The function of the touchpad is realized by the projector-camera system.