Molten salt reactors (MSRs) are liquid-fueled reactors that adopt molten salt, dissolved with the fissile and fertile elements, as the fuel and coolant. During the core operation of MSRs, the sparingly soluble gaseous fission products (GFPs) will be transported with molten salt through the primary loop, involving migration among the molten salt, helium bubbles, and graphite/alloy pipes. Accurate characterization of the migration behavior of GFPs is essential for xenon poisoning control, safety assessment, and isotope production evaluation in MSRs. In this study, a gaseous fission products migration behavior model was developed for MSRs. The concentration of typical gaseous fission product, Xe-135, in the molten salt, circulating bubbles, and graphite pores during the transients of reactor startup and shutdown was then analyzed by using the developed model. The sensitivity of void fraction, circulating bubble size, component temperature, and molten salt viscosity to the migration behavior of Xe-135 was finally investigated. This study will provide references for the design and safety analysis of MSRs.