Single-crystal diamond (SCD) detectors promise to have applications in neutron spectrometers and fusion neutron monitoring under high flux deuterium plasma. The response to 14 MeV neutrons for the SCD detector is studied in this paper. A high-performance SCD neutron detector is developed by processing cleaning wafer, depositing metal electrodes, annealing and wire-bonding. A fast-neutrons monitoring system containing the detector, preamplifier and digital multichannel analyzer is constructed, and the response to 14 MeV neutrons for the detector is measured on the K-400 neutron generator supported by China Academy of Engineering Physics. In addition, computational simulations of the energy deposition and detection efficiency of 14 MeV neutron through diamond are performed via Geant4 toolkit based on evaluated nuclear data libraries of ENDF-VIII.0, JEFF-3.3, BROND-3.1, JENDL-4.0u and CENDL-3.1. The methods of widening the simulation spectrum and calibration of measuring spectrum are presented in order that simulation results are in reasonable agreement with measured values. The results indicate that the energy deposition of 14 MeV neutrons incident on the ¹²C can be more accurately calculated with CENDL-3.1 than with other data libraries. The elastic scattering and reaction of ¹²C(n, 3α) are described more accurately with the CENDL-3.1, and the characteristic peaks of ¹²C(n, α)⁹Be matched well the calibrated testing spectrum and the after-widening simulation spectrum, with a difference between the simulated detection efficiency and measuring results being as low as 0.61%. The outcome measures are described as the standardized mean difference, with a detection efficiency of (3.31 × 10^–4 ± 0.11 × 10^–4) counts/n, an energy resolution of 4.02% ± 0.09%, and a peaking channel of 1797.24 ± 0.80, which suggest that the detector keeps stable well under a high neutron flux of 2 × 10¹⁰ n/s for as long as 2 h. The results demonstrate that the SCD detector can be a promising candidate for monitoring 14 MeV D-T neutrons.