The entanglement dynamics in a two-qubit Rabi model is discussed by the extended coherent states (ECS) method. The two qubits are initially prepared in an exchange-symmetric Bell state and the initial state of the light field is the vacuum state. The entanglement evolution characteristics are analyzed under different transition frequencies of qubits and different coupling strengths between light fields and qubits. The results show that, in the case of weak coupling, when the differences between the transition frequencies of two identical qubits and the frequency of the light field are equal, the entanglement evolution is almost identical. When the transition frequencies of two non-identical qubits are symmetrically detuned relative to the frequency of the light field, the entanglement degree is larger than that for two identical qubits; the larger the detuning is, the stronger the entanglement is, and the entanglement evolution period has an inverse relationship with the detuning. Under the resonance conditions and when the coupling strengths are different, the phenomenon of the principal peaks and the secondary peaks appearing alternately in the entanglement evolution process of two qubits. If the coupling strength between one qubit and the light field is kept to be constant, the stronger the coupling strength of the other qubit is, the higher the secondary peaks become but, the principal peaks always reach the maximum entanglement. The entanglement evolves periodically in the whole process.