Lanthanum bromide detector is a new kind of scintillation detector appeared recent years. Due to the better energy resolution and detection effective

it is developed rapidly and used widely. This paper calculates and researches the characteristic of γ spectrum of the lanthanum bromide detector. The calculation is based on Geant4

the general purpose Monte Carlo program package

and compared with the experiment data. It demonstrates that the lanthanum bromide detector can be used in portable γ spectrometer. The calculation also provides reliable data under limited experiment for the development of spectral analysis algorithm.