The dynamics of two qubits with inter-qubit interaction is investigated by an external quantum oscillator in its coherent state. The coupling between the two qubits and the oscillator is in the ultra-strong region and their detuning is large. With adiabatic approximation method, the reservation of one initially entangled state of two qubits by the quantum oscillator is accomplished through the careful choice of the four parameters, the average number of the coherent state of oscillator, the ultra-strong coupling strength, the ratio of two frequencies of qubit and oscillator, and the inter-interaction coupling of two qubits. If the inter-qubit interaction strength is negative, their entanglement can be kept further for a much longer time. These results are different from those in the previous studies and reasonable explanations are given. Maintaining the entanglement of two qubits with very high fidelity will be helpful for the quantum information process.