  The operation of AMS-02 opens a new era for the study of cosmic ray physics
with unprecedentedly precise data which are comparable with the laboratory
measurements. The high precision data allow a quantitative study on the cosmic
ray physics and give strict constraints on the nature of cosmic ray sources.
However, the intrinsic errors from the theoretical models to interpret the data
become dominant over the errors in the data. In the present work we try to give
a systematic study on the uncertainties of the models to explain the AMS-02
positron fraction data, which shows the cosmic ray $e^+e^-$ excesses together
with the PAMELA and Fermi-LAT measurements. The excesses can be attributed to
contributions from the extra $e^+e^-$ sources, such as pulsars or the dark
matter annihilation. The possible systematic uncertainties of the theoretical
models considered include the cosmic ray propagation, the treatment of the low
energy data, the solar modulation, the $pp$ interaction models, the nuclei
injection spectrum and so on. We find that in general a spectral hardening of
the primary electron injection spectrum above $\sim50-100$ GeV is favored by
the data. Furthermore, the present model uncertainties may lead to a factor of
$\sim2$ enlargement in the determination of the parameter regions of the extra
source, such as the dark matter mass, annihilation rate and so on.
