1. A displacement detection method of detecting displacement of an optical mouse navigation device with respect to a working surface, the displacement detection method comprising: acquiring a first image frame with an image sensor; calculating, using a processing unit, a first characteristic index of the first image frame; comparing, using the processing unit, the first characteristic index with a threshold value; calculating, using the processing unit, a displacement according to the first image frame when the first characteristic index is larger than the threshold value; adding, using the processing unit, a fixed pattern to the first image frame to form a second image frame when the first characteristic index is smaller than the threshold value; calculating, using the processing unit, a second characteristic index of the second image frame; comparing, using the processing unit, the second characteristic index with the threshold value; and calculating, using the processing unit, a displacement according to the second image frame when the second characteristic index is larger than the threshold value.