In condensed matter and many-body physics, fidelity is often used to detect quantum phase transitions in parameter space. The definition of fidelity is the inner product of the ground state wave functions of two adjacent points in parameter space, F = | ⟨ ⟨ ψ ψ 0 ( λ λ ) | ψ ψ 0 ( λ λ + ϵ ϵ ) ⟩ ⟩ | 2 , where ϵ ϵ is a small quantity. After series expansion, F = 1 − − χ χ F ϵ ϵ 2 + O ( ϵ ϵ 3 ) , the first-order correction term of fidelity is zero, and the coefficient of the second-order correction term is called the fidelity susceptibility. The fidelity susceptibility diverges toward positive infinity as the parameters approach the quantum phase transition point.