Touch devices have become one of the major elements in today's most electronic products. Touch technology can be divided into different types as resistive, capacitive, infrared, and so on. However, as the amount of detection sensors increases, most types of touch screen suffer from more cost and lower yield when the accuracy of touch location or larger size is required. Therefore, it is essential to design a different kind of touch technology with accurate detection and the cost is not proportional to its size. This paper proposed a kind of touch system using the distance between the object (eg. finger or stylus) and its shadow to determine whether the object touched the screen or not, and locate the touch point. It can be applied to any interactive projection system without using a lot of touch detecting sensor. Therefore, no additional costs are required for a large touch screen. The proposed touch system only requires one camera and one IR source, and cooperates with an interactive projector to detect the occurrence of touching and its location. The proposed system achieves an average detection rate of 97.53% if the tolerance of detected touch coordinates is in ± 10 pixels.