1. A target detection method comprising the following steps: controlling a modulated light emitting device to emit light pulse signals with a first light intensity and a second light intensity to a target to be detected and a background, wherein the first light intensity is not equal to the second light intensity, and the capabilities of reflecting the light pulse signals of the target to be detected and the background are different; controlling an image sensor to acquire images of the target to be detected and the background, wherein the image sensor comprises a plurality of image acquisition regions, wherein two consecutive scans are performed on the same image acquisition region respectively in the first light intensity and in the second light intensity to obtain a first light intensity image and a second light intensity image which are then stored in corresponding locations in a first frame image and a second frame image respectively; obtaining the first frame image and the second frame image after finishing the scanning for the plurality of image acquisition regions and distinguishing the target to be detected and the background by means of the first frame image and the second frame image; in which regional grey scales for the target to be detected imaged in the first light intensity and in the second light intensity are quite different and regional grey scales for the background imaged in the first light intensity and in the second light intensity have little difference so that the distinguishing the target to be detected and the background is achieved through a difference image obtained by subtracting the first frame image from the second frame image.