Both Boltzmann distribution method (BDM) and mean square displacement method (MSDM) are available for the force calibration of non-spherical biological cells in a harmonic optical trap. These two calibration methods were compared with numeric experiments. Results indicate that comparing with MSDM,BDM is not only available for the force calibration of nonspherical or anisotropic cells,but also for irregular cells in optical trap potential with inharmonic and asymmetric profiles. The results agree with the experiments reported.