In the analysis of combustion instabilities, especially for the longitudinal modes, the flame typically is regarded as a discontinuity at rest and the flame motion is usually ignored. However, this may lead to errors in the prediction of combustion instabilities for certain conditions. This paper investigates the effect of flame motion on the longitudinal combustion instabilities in a simple ducted combustion chamber. An analytical model considering the flame motion is incorporated into the thermoacoustic system. The dispersion relation is established, and the eigenvalues of thermoacoustic system can be obtained by solving it. The present study considers the cases with and without entropy noise produced at the outlet boundary. The effect of flame motion on longitudinal combustion instabilities is evaluated by consider it and not. When there is no entropy noise, the effect of the flame motion remains small. However, when the entropy noise is considered, the flame motion can have a large effect on different longitudinal modes. Furthermore, the presence of the flame motion can dissipate the entropy wave generated from the flame and further the corresponding combustion instability for the present configuration.