  The S-wave $\pi - \pi$ scattering lengths $a_0$ and $a_2$ are calculated to
the lowest order of $1/N_c$ expansion in the general framework of SU(2) NJL
model beyond mean-field approximation. It is shown that using the universal
curve of $a_0$ and $a_2$ the four NJL parameters, i.e., the current quark mass
$m_0$, the four fermion coupling constant $G$, the quark momentum cut-off
$\Lambda_f$ and the meson momentum cut-off $\Lambda_b$, also the S-wave $\pi -
\pi$ scattering lengths in NJL model can be uniquely determined.
