  We present a detailed analysis of spin squeezing of the one-axis twisting
model with a many-body phase dephasing, which is induced by external field
fluctuation in a two-mode Bose-Einstein condensates. Even in the presence of
the dephasing, our analytical results show that the optimal initial state
corresponds to a coherent spin state $|\theta_{0}, \phi_0\rangle$ with the
polar angle $\theta_0=\pi/2$. If the dephasing rate $\gamma\ll S^{-1/3}$, where
$S$ is total atomic spin, we find that the smallest value of squeezing
parameter (i.e., the strongest squeezing) obeys the same scaling with the ideal
one-axis twisting case, namely $\xi^2\propto S^{-2/3}$. While for a moderate
dephasing, the achievable squeezing obeys the power rule $S^{-2/5}$, which is
slightly worse than the ideal case. When the dephasing rate $\gamma>S^{1/2}$,
we show that the squeezing is weak and neglectable.
