This paper focuses on fast algorithm for computing the assignment reduct in inconsistent incomplete decision systems. It is quite inconvenient to judge the assignment reduct directly according to its definition. We propose the judgment theorem for the assignment reduct in the inconsistent incomplete decision system, which greatly simplifies judging this type reduct. On such basis, we derive a novel attribute significance measure and construct the fast assignment reduction algorithm(FARA), intended for computing the assignment reduct in inconsistent incomplete decision systems. Finally, we make a comparison between FARA and the discernibility matrixbased method by experiments on 13 University of California at Irvine(UCI) datasets, and the experimental results prove that FARA is efficient and feasible.