  The reaction mechanism of the central collisions and peripheral collisions
for $^{112,124}Sn+^{112,124}Sn$ at $E/A=50MeV$ is investigated within the
framework of the Improved Quantum Molecular Dynamics model. The results show
that multifragmentation process is an important mechanism at this energy
region, and the influence of the cluster emission on the double n/p ratios and
the isospin transport ratio are important. Furthermore, three observables,
double n/p ratios, isospin diffusion and the rapidity distribution of the ratio
$R_{7}$ for $^{112,124}Sn+^{112,124}Sn$ at E/A=50MeV are analyzed with the
Improved Quantum Molecular Dynamics model. The results show that these three
observables are sensitive to the density dependence of the symmetry energy. By
comparing the calculation results to the data, the consistent constraint on the
density dependence of the symmetry energy from these three observables is
obtained.
