The world’s first 550 MWe ultra-supercritical circulating fluidized bed (CFB) boiler designed with integrated recycle heat exchangers (INTREX) commenced operation in 2016. One of the distinguishing features of the INTREX is the internal circulation opening (ICO) that allows the particles in the furnace to flow into the INTREX to enhance the bed-to-tube heat transfer, while the heat transfer is strongly affected by both the mass flow rate of internal circulation m˙ i c and particle size in the INTREX, ie, the mass balance of the INTREX. In the present study, we established a comprehensive one-dimensional model to investigate the mass balance of the boiler including the internal circulation. The coefficient of rebound η I C O was adopted to simulate particle behavior during internal circulation. The experimentally acquired parameters of ash formation and attrition of the coal ash were adopted in the model. The size of INTREX ash obtained from the field test on the 550 MWe boiler was used to determine the value of η I C O. The model results were verified with field tests, and they showed reasonable validity. The model results of the linear correlation between m˙ i c and bed pressure displayed good agreement with those from the field tests.