Neutron-TPC (nTPC) is a fast neutron spectrometer based on GEM-TPC (Gas Electron Multiplier-Time Projection Chamber) and expected to be used in nuclear physics, nuclear reactor operation monitoring, and thermonuclear fusion plasma diagnostics. By measuring the recoiled proton energy and slopes of the proton tracks, the incident neutron energy can be deduced. It has higher n/gamma separation ability and higher detection efficiency than conventional neutron spectrometers. In this paper, neutron energy resolution of nTPC is studied using the analytical method. It is found that the neutron energy resolution is determined by 1) the proton energy resolution (sigma(Ep)/E-p), and 2) standard deviation of slopes of the proton tracks caused by multiple Coulomb scattering (sigma(k(scattering))) and by the track fitting accuracy (sigma(k(fit))). Suggestions are made for optimizing energy resolution of nTPC. Proper choices of the cut parameters of reconstructed proton scattering angles (theta(cut)), the number of fitting track points (N), and the working gas help to improve the neutron energy resolution.