A new type of touchpad based on the optical sensitive device PSD is designed to avoid the disadvantages on the property and the cost of the present interactive touchpad system.The optical system consists of reflectors and other optical devices.It makes images of the light from the touch pen which locates on the surface of the touchpad and finally images on the optical sensitive device PSD seated on the back of the touchpad.That can determine the position coordinates of the touch pen.The optical sensitive device PSD is adopted to enhance the speed of data processing.The structure of system is much simpler and also easier to be integrated.The low cost and the superior property also contributes to the application of big size and large scale touchpad.