A system for obstacle detection adapted to a self-guiding machine is provided. The system includes a controller, a linear light source and a light sensor. The linear light source and the light sensor are set apart at a distance. When the linear light source emits an indicator light being a vertical linear light projected onto a path the self-guiding machine travels toward, the light sensor senses the indicator light. The vertical linear light is segmented into a first segment projected to a ground and a second segment projected to a floating obstacle when the self-guiding machine approaches the floating obstacle with a height from the ground and the indicator light is projected to the floating obstacle, in which the second segment of the light sensed by the light sensor is determined as the floating obstacle in front of the self-guiding machine.