Abstract: A formula for estimating the detuning effect in the interlaced side coupledstanding wave accelerator(IL) is deduced by considering the interaction betweenthe electron beam and the electric fields in the gaps of cylindrical symmetry: ΔE/E=-1/2(1+e~(-1,6/N-1))Q_L~2(f_(01)-f_(02)/f_m)~2 It is proved by particle dynamic calculations using a computer and the resultsare compared to other works.