1. A method for banknote identification based on thickness signal identification, comprising: acquiring a thickness signal of a banknote; preprocessing the thickness signal; identifying the thickness signal using an upward area identification method to acquire an upward processing identification result; identifying the thickness signal using a downward area identification method to acquire a downward processing identification result; merging the upward processing identification result and the downward processing identification result according to a preset rule to acquire a merging result; and identifying the merging result to acquire an identification result; wherein the identifying the thickness signal using an upward area identification method comprises: taking an area of a region formed by a sliding window, an upper threshold line and a curve of the thickness signal as a thickness feature upper area, wherein a width and a sliding step of the sliding window have preset values, and the upper threshold line is a horizontal line above the curve of the thickness signal; moving the sliding window in a stepwise manner with the preset sliding step and calculating thickness feature upper areas at all steps; acquiring a minimum thickness feature upper area among the thickness feature upper areas calculated at all the steps; calculating an average thickness feature upper area of the thickness feature upper areas calculated at all the steps; and identifying the thickness signal according to the minimum thickness feature upper area and the average thickness feature upper area to acquire the upward processing identification result; wherein the identifying the thickness signal using a downward area identification method comprises: taking an area of a region formed by a sliding window, an lower threshold line and a curve of the thickness signal as a thickness feature lower area, wherein a width and a sliding step of the sliding window have preset values, and the lower threshold line is a horizontal line below the curve of the thickness signal; moving the sliding window in a stepwise manner with the preset sliding step and calculating thickness feature lower areas at all steps; acquiring a minimum thickness feature lower area among the thickness feature downward areas calculated at all the steps; calculating an average thickness feature lower area of the thickness feature lower areas calculated at all the steps; and identifying the thickness signal according to the minimum thickness feature lower area and the average thickness feature lower area to acquire the downward processing identification result.