Abstract: In the quantum many-body system, the calculation of the ground state is the key target problem. Variational quantum eigensolver (VQE) is a variational ground state solution algorithm based on quantum computation. However, since it requires combining quantum circuit and classical variational algorithm, the complexity of the quantum circuit and the choice of variational algorithm becomes extremely important. This paper focuses on quantum molecular systems and proposes a variational ground state solver. It uses the single-electron reduced density matrix analysis to obtain the number of electron occupations under the appearance of natural molecular orbitals. According to the number of occupations, the Hamiltonian of the system and the corresponding unitary couple cluster (UCC) ansatz circuit are greatly simplified. Secondly, the variational quantum imaginary time evolution algorithm is used to replace the commonly used gradient algorithm in VQE, which is not easily influenced by the gradient distribution of the parameter space, causing the variational process to converge more quickly and robustly.