The invention provides a method for determining the angular magnitude of an imaging acquiring apparatus, which is capable of obtaining an arranging angular magnitude with respect to the imaging acquiring apparatus in various environments by an iteration algorithm having parameters of position of a calibrating tool with respect to an image coordinate system and characteristic value corresponding to a spatial coordinate system respectively. Meanwhile, the present further provides a vehicle collision warning system which functions to obtain the relative distance between the carrier and objects around according to the image acquired by the imaging acquiring apparatus and the angular magnitude and height of the imaging acquiring apparatus. By means of the method disclosed in the present invention, the procedure to detect the angular magnitude of the image acquiring apparatus is more convenient and accurate such that the convenience for using the vehicle collision warning system is capable of being improved.