A method and a system of detecting a tilt angle of an object surface and a method and a system of compensating the same are provided. The detecting method includes the following steps. Light beams are projected by a light source device to the object surface. An image of the object surface is captured so as to obtain light spots on the object surface. A focus program is executed by adjusting a vertical distance between the light source device and the object surface, so as to gather the light spots in a focal point on the object surface. The vertical distance is adjusted, and a correction angle between the light beams and the object surface is calculated according to the light spots.