1. A resolution adjusting method, applicable to an image capturing device, wherein the image capturing device comprises a camera module and a storage unit, the resolution adjusting method comprising: capturing an image with a first resolution by the camera module; analyzing an amount of objects in the image; comparing the amount of the objects with at least one threshold value; when the amount of the objects is smaller than at least one of the at least one threshold value, lowering the resolution of the image from the first resolution to a second resolution according to the at least one threshold value which the amount of the objects is smaller than, and storing the image with the second resolution in the storage unit; and when the amount of the objects is greater than or equal to all the threshold value, storing the image with the first resolution in the storage unit.