The invention relates to a colloidal gold detecting method based on digital image processing, comprising the following steps of: collecting RGB (Red, Green, Blue) coloured images by an area array CCD (Charge Coupled Device) detector, carrying out gray level conversion, edge extraction, background suppression and the like on the collected coloured images, and finally quantitatively calculating thecorresponding concentration result of a sample. The invention also relates to a colloidal gold detecting device based on the digital image processing; and the device can be used for detecting the colloidal gold by the method provided by the invention so that the device not only realizes the negative and positive qualitative judgement of the sample but also realizes the quantitative application onthe lower end.