Based on the isospindependent quantum molecular dynamics model, we have invest igated the dependences of isospin fractionation degree (N/Z)n/(N/Z)fragon the mass and the beam energy of colliding system. The (N/Z)n a nd (N/Z)frag are the neutron proton ratio of the nucleon emission (gas-phase) and that of the fragment emission (liquid phase) respectively. The isosp in fractionation degree is a sensitive func tion to the mass and beam energy of the colliding system. The (N/Z)n/(N/Z)frag reduces with increasing mass of the colliding system when the beam energy and n eutron proton ratio of the colliding system are fixed. This is due to the larg er compr ession, small heating energy and larger critical temperature of the gasliquid phase transition for the heavy colliding system, compared to those of the light colliding system. We also found that (N/Z)n/(N/Z)frag enhances wit h increasing beam energy due to the increase of excitation energy of the collidi ng system which leads to larger nucleon transport and isospin fractionation. But the isospin frac tionation only occurr in the energy region for producing the liquidgas phase t r ansition. In this case, we propose that (N/Z)n/(N/Z)frag can be directly compa red with the experimental data to get the information about symmetry potential.