Modulating La-site defects is an efficacious method to boost the catalytic activity of LaMnO3 for flue gas purification. Herein we fabricated various La1-xMnO3 (x = 0–0.4) with different non-stoichiometry of La-ions, conducted Hg0 removal experiments and discussed the effects of La-site regulation on the physical–chemical properties and adsorption properties of perovskites. Compared with pristine LaMnO3, the catalytic activity of modulated La1-xMnO3 was improved, and La0.85MnO3 exhibited optimal Hg0 removal performance with a removal efficiency of up to 99.6 %, where Hgads and Hgesp were determined as the prevalent existing forms of Hg-species as results of chemisorption and oxidation. Characterizations and theoretical calculations clarified that: (i) after constructing La-defects, the structural properties, the amounts of Mn(IV) and oxygen vacancies were improved; (ii) the modulation shortened the Mn-O bond, strengthened the hybridization between Mn 3d and O 2p orbitals, elevated the valence state of Mn, and activated lattice oxygen; (iii) the defective surface benefited the O2 activation, forming monatomic oxygen and replenishing lattice oxygen, which were major adsorption sites for Hg0 oxidation; (iv) the electron transfer among O2, oxygen vacancies and Mn(IV)/Mn(III) contributed to the removal. This nonstoichiometric modulation favors the construction of defective perovskites, laying the foundation for rational development of high-performance catalysts.