1. An optical touch system comprising: a first imaging device being directed toward a side surface of a target on a touch surface, and generating, in sequence, a first picture comprising an image of the target and a second picture comprising an image of the target; and a computing device configured to calculate a first dimension that represents the target at the time when the first picture is taken using the first picture, and a second dimension that represents the target at the time when the second picture is taken using the second picture, and to determine whether the target is formed by one object or two objects by the first and second dimensions, wherein the computing device is configured to compare the first dimension with the second dimension, wherein the computing device is configured to calculate a ratio of the first dimension to the second dimension and to compare the ratio with a threshold.