The Nab experiment is designed to a) determine the ratio of axial-vector coupling to vector coupling to an uncertainty of 0.03% error, b) test the unitarity of CKM Matrix and c) measure the Fierz interference term b by studying free neutron beta decay. In neutron beta decay, the electron energy spectrum is modified by the Fierz interference term. Therefore, a precise measure of the electron energy spectrum could lead to an excellent determination of b. The Nab experiment is expected to measure the Fierz interference term with an uncertainty of about 10^-3. We present the uncertainty analysis of the electron energy spectrum, and show the required limit of the experiment uncertainty in different aspects.