Taking the long-wavelength Rayleigh-Taylor instability (RTI) on the thin shell of inertial confinement fusion as the research object,a linear analytical model is presented to study the phase effects that are caused by the phase difference of single-mode perturbations on the two interfaces.Its accuracy is tested by numerical simulations.By analyzing the characteristic of this model,it is found that the phase difference does not change the basic RTI structure (only one spike and one bubble in a period).However,the symmetry of the spike and bubble is destroyed,which has non-expected influences on the convergent motion of ICF targets.Meanwhile,the phenomenon that the distance between spikes and bubbles along the vertical direction of acceleration differs by π is demonstrated.It is also shown that when the phase difference is large,the temporal evolution of the RTI is more serious and the thin target is easier to tend to break.