An optical touch system including a touch surface, a reflection structure and light source modules is provided. The reflection structure and the light source modules are disposed at a periphery of the touch surface. Each light source module has a light-emitting unit and a sensing unit. The sensing unit obtains a brightness value distribution of a sensing light emitted by the light-emitting unit after the sensing light is reflected by the reflection structure, so as to sense touch input. The periphery of the touch surface includes first and second sections. When the light-emitting unit located on the first section emits the sensing light, the sensing light is not reflected by the reflection structure at the light source modules located on the second section so that low-brightness areas are formed, and the light-emitting units of the second group emit compensation lights correspondingly. In addition, a touch sensing method is also provided.