  In this paper, we tentatively assign $Z_{c}(3900)$ to be an axialvector
molecular state, and calculate its magnetic moment using the QCD sum rule
method in external weak electromagnetic field. Starting with the two-point
correlation function in external electromagnetic field and expanding it in
power of the electromagnetic interaction Hamiltonian, we extract the mass and
pole residue of $Z_{c}(3900)$ state from the leading term in the expansion and
the magnetic moment from the linear response to the external electromagnetic
field. The numerical values are $m_{Z_{c}}=3.97\pm0.12\mbox{GeV}$ in agreement
with the experimental value $m^{exp}_{Z_{c}}=3899.0\pm3.6\pm4.9\mbox{MeV}$,
$\lambda_{Z_{c}}=2.1\pm0.4\times10^{-2}\mbox{GeV}^{5}$ and
$\mu_{Z_{c}}=0.19^{+0.04}_{-0.01}\mu_{N}$.
