A landmine detection system based on D-T sealed neutron generator was set up to verify the validity of fast neutron analysis (FNA) for landmine detection. The results of experiment and MCNP5 simulation prove that the capacity of FNA for landmine detection is greatly limited due to its high background radiation levels. Inspired by previous efforts, the application of tagged neutron method (TNM) for landmine detection was investigated and modeled by MCNPX code. The gamma spectra of graphite sample were compared at different time windows. The appropriate selection of time window is effective for the landmine location and the background suppression. The influence of the soil humidity was studied for the landmine buried at different depths. The simulation results showed that the peak area ratio of C/Si and C/O can be used to effectively identify explosive materials in the detection area.