Abstract: An X-band detuned accelerator structure was designed using the Mafia code and the parameters were calculated efficiently through the Lagrange interpolation method, then the characteristics and wakefield of this structure were obtained in the end. The results indicate that this structure not only has high quality factor and high shunt impendence, but also can suppress the wakefield to decrease significantly to a low level comparing with the constant impedance accelerator structure, so that the probability of beam break-up is reduced to a great extent.