Based on the high dielectric tunability, high dielectric constant and small dielectric loss at paraelectric phase, lead-free relaxor ferroelectrics have been used in microwave devises widely. According to the present theory of dielectric tunability, the expression of dielectric tunability is derived by adjusting parameters properly. The derived expression can be used to deal with experimental results. It is found that there exists a great difference between experimental result and the theoretical result obtained under the assumption of unchanged paraelectric phase under an electric field, while the experimental result and the theoretical result of Johnson in 1962 is consistent. A modified relation of dielectric constant with higher electric field is derived by considering the Gibbs free energy modified with an external electric field and a spontaneous polarization. The result from the modified equation is in agreement with the experimental result. A concept of degree of tunability is proposed to express quantitatively the relationship between dielectric tunability and concentration of dopants.