DFT-B3LYP/6-311G** method has been used to optimize molecular geometric structures of 527 polychlorinated fluoranthene (PCFRT) congeners and calculate their thermodynamic properties in the ideal gas state, such as heat capacity at constant volume (C^θ_v), entropy (S^θ), standard enthalpy of formation (Δ_fH^θ) and standard Gibbs free energy of formation (Δ_fG^θ）The relations of C^θ_v, S^θ，Δ_fH^θ and Δ_fG^θ with the number and position of chlorine atoms have also been explored, from which the relative stability of PCFRT congeners was theoretically proposed according to the magnitude of the relative standard Gibbs free energy of formation (Δ_r,_fG^θ). It was found that some of the PCFRT isomers are in a nonplanar configuration and there are intramolecular Cl···Cl weak interactions in some of the isomers. With increasing the number of chlorine atoms, the values of Δ_fH^θ and Δ_fG^θ of the most stable PCFRT isomers decrease initially and then increase. The values of Δ_fH^θ and Δ_fG^θ of PCFRT congeners with the same number of chlorine atoms show a strong dependence on the positions of chlorine atoms. The relative thermodynamic stability of PCFRT isomers is determined mainly by intramolecular adjacent Cl···Cl nuclear repulsive interaction between Cl atoms at two different six-membered rings. Most PCFRT congeners are easier to form thermodynamically than their parent compound.