1. An image capturing device capable of simplifying characteristic value sets of captured images, comprising: a characteristic conversion unit, for converting a captured image into a characteristic image which includes a group of first characteristic value sets; a data storage unit, for storing a lookup table which comprises second characteristic value sets; a characteristic simplification unit, for executing a simplification process according to the lookup table, and deleting the first characteristic value set not existing in the lookup table from the group of first characteristic value sets, and replacing each deleted first characteristic value set by the second characteristic value set matching with a predetermined condition and existing in the lookup table, so as to generate a simplified group of characteristic value sets; a template storage unit, for storing a plurality of templates; and a recognition unit, for comparing the simplified group of characteristic value sets with the plurality of templates to recognize a specific object in the captured image; wherein the predetermined condition is determined by differences between the deleted first characteristic value set and the second characteristic value sets, and the deleted first characteristic value set is replaced by the second characteristic value set with the least difference, and the lookup table is established by using a characteristic value set with entropy less than a threshold value of an entropy of the characteristic value set.