Abstract: As the marine nuclear power device is operated widely, the thermal hydraulic phenomenon caused by motion conditions is considered significantly. Since many parallel channels exist in the reactor core, the study of flow instability in parallel double channels under single and coupled motion conditions was carried out by modifying the source term in momentum equation adopted in RELAP5 code. The marginal stability boundary (MSB) curves under static and motion conditions were compared, and it is shown that the effect of different motion conditions on flow instability is very small under forced circulation. With the same subcooled number (N_sub), the difference between phase change numbers (N_pch) of static condition and motion condition is under 2%.