The reaction mechanism of the central collisions and peripheral collisions is investigated firstly for 112,124Sn+112.124Sn at E/A=50MeV for different impact parameters 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 the ratio R7 for 112,124Sn+112,124Sn 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 these calculations to the data, the consistent constraint on the density dependence of the symmetry energy from these three observables is obtained.