Band gap anomaly is a well-known issue in lead chalcogenides Pb X(X = S, Se, Te, Po). Combining ab initio calculations and tight-binding(TB) method, we have studied the band evolution in Pb X, and found that the band gap anomaly in Pb Te is mainly related to the high on-site energy of Te 5s orbital and the large s–p hopping originated from the irregular extended distribution of Te 5s electrons. Furthermore, our calculations show that Pb Po is an indirect band gap(6.5 me V) semiconductor with band inversion at L point, which clearly indicates that Pb Po is a topological crystalline insulator(TCI). The calculated mirror Chern number and surface states double confirm this conclusion. Banding anomaly is a well-known issue in lead chalcogenides Pb X (X = S, Se, Te, Po). Combining ab initio calculations and tight-binding (TB) method, we have studied the band evolution in Pb X, and found that the band gap anomaly in Pb Te is mainly related to the high on-site energy of Te 5s orbital and the large s-p hopping originated from the irregular extended distribution of Te 5s electrons. an indirect band gap (6.5 me V) semiconductor with band inversion at L point, which identifies that Pb Po is a topological crystalline insulator (TCI). The calculated mirror Chern number and surface states double confirm this conclusion.