A sequential decay model is used to study isoscaling, I.e. The factorization of the isotope ratios from sources of different isospins and sizes over a broad range of excitation energies, into fugacity terms of proton and neutron number, R21(N, Z) = Y2(N, Z)/Y1(N, Z) = Cexp(αN +βZ). It is found that the isoscaling parameters α and β have a strong dependence on the isospin difference of equilibrated source and excitation energy, no significant influence of the source size on α andβ has been observed. It is found that α and β decrease with the excitation energy and are linear functions of 1/T and △(Z/A)2 or △(N/A)2 of the sources. Symmetry energy coefficient Csym is constrained from the relationship of α and source △(Z/A)2, β and source △(N/A)2.