The neutron emission spectra associated with fusion reactions contain valuable information on both the bulk plasma and fast-ion behaviour before the reaction, including information, e.g., energy and momentum distribution, on another fusion product after the reaction, e.g., α-particle for T(d,n)α reaction. Anisotropic fuel-ion velocity distribution functions owing to anisotropic beam injection and/or recoil component (referred to as the knock-on tail) production via nuclear elastic scattering (NES) cause anisotropic deviation of the neutron emission spectrum from the typical Gaussian distribution function. In this investigation, the anisotropic slowing-down distribution functions of the deuterium beam (injected tangentially or perpendicularly to a toroidal magnetic field) and the recoil deuterons produced by NES when a tangential hydrogen beam was injected were evaluated using a 3D particle trajectory simulation, assuming JT-60SA relevant deuterium plasma. Using the obtained deuteron distribution functions, the double-differential neutron emission spectra from D(d,n)3He reactions were evaluated as the average value within the volume enclosed by magnetic flux surfaces as a function of the poloidal radius. The neutron currents that reached the neutron spectrometer per unit surface and time were estimated in several directions from the double-differential neutron emission spectra. Moreover, the neutron spectra for various directions of sight were obtained. It was demonstrated that the neutron spectrum from a Gaussian distribution, resulting from knock-on tail formation in the deuteron distribution function due to the injection of a 500 keV hydrogen beam, falls within the measurable range for the JT-60SA deuterium plasma, despite suitable plasma conditions should be prepared.