The intrinsic low lattice thermal conductivities originates from the complex anion bonding environment make quaternary chalcogenides potential thermoelectric materials. Here, the bonding environment of Se atom in Cu_2.1Mn_0.9SnSe₄ is further regulated by substituting Mn²⁺ with equimolar pairing elements (Ag⁺ and In³⁺). The increase in both bond length and angle, together with the reduction in bond strength of Ag–Se and In–Se bonds, cause the doped samples to display strong anharmonicities (γ ~ 1.84–2.04). And the weakened bond strength also lower the sound velocities. Consequently, the κ_L of the doped samples is effectively constrained, achieving a minimum value of 0.55 W·m⁻¹·K⁻¹ at 673 K in x = 0.10 sample. Ultimately, a zT value of 0.53 at 673 K is attained in x = 0.10 sample. The modification of bonding environment around anion is considered as an effective mean to optimize the thermoelectric performance of quaternary chalcogenides.