1. An image analysis device comprising: an image capturing module configured to obtain an image of an scene; a brightness calculating module, configured to obtain at least one brightness value of at least one area of the image obtained by the image capturing module, and obtain a brightness of the same at lease one area of each of a predetermined number of previous images of the same scene obtained prior to the image and calculate an average brightness of the same at lease one area of the predetermined number of previous images to obtain at least one average brightness value; a brightness comparison module, configured to compares the at least one brightness value of the image with the at least one average brightness value of the predetermined number of previous images to obtain at least one brightness difference value, and compare the at least one brightness difference value with a first value and a second value greater than the first value; and a background model adjustment module, configured to adjust the reference background model according to a first adjustment mode when the comparison module determines a majority of the at least one brightness difference value is greater than the first value and is less than the second value, and adjust the reference background model according to a second adjustment mode when the comparison module determines a majority of the at least one brightness difference value is equal to or greater than the second value.