Abstract: Energy level density plays an important role in the study of kinetic and thermodynamic properties of nuclear physics. In this paper, shell model and the truncation algorithm are used to calculate the lowest 10 000 states of Sn isotopes. The number of holes and the number of particles in the ^116\rmSn isotope of Sn is 8, and the energy level is the most complex. According to the ^116\rmSn energy level density, the relationship between energy level density and angular momentum and the microcanonical entropy of the system are studied. It is found that the lowest average energy level under even parity has an obvious odd-even effect of angular momentum, which can be reasonably explained by Pauli exclusion principle. Furthermore, the properties of different Sn isotopes are studied and a similiar conclusion is obtained. The microcanonical entropy of ^116\rmSn is fluctuated by odd parity in the low energy band, which is related to the fracture of neutron pairs.