The invention provides an image capturing device angle deciding method and a vehicle collision warning system thereof, which can calculate with the characteristics of a calibration tool about the position of an image coordinate system and a space coordinate system by considering the recursive angle in a plurality of environments, so as to obtain a precise image capturing device angle. The method comprises that an image capturing device is arranged on a carrier, so that the image capturing device has a preset angle scope; and the image capturing device captures an image of the calibration tool, to respectively calculate the position of the image coordinate system of the image which is characterized by the image capturing device and a characteristic about the space coordinate system, carries out a recursive angle calculation procedure is carried out according to the position and the characteristic, so as to obtain the set angle of the image capturing device. Through the detection method, the angle of detecting the image capturing device becomes simple and accurate, so as to improve the use convenience of the vehicle collision warning system.