It is necessary to obtain the triggering boundaries of heat transfer deterioration by mastering the flow and heat transfer characteristics in plate-type fuel assembly with multiple channels blocked, to improve the operation safety of plate-type fuel reactors. Based on qualitative analysis, the flow channel partial blockage accidents can be divided into non-adjacent channel blockage accident and adjacent channel blockage accident for the standard fuel assembly of the typical plate-type fuel reactor JRR-3M. Furthermore, the simulations of the flow and heat transfer characteristics under the two types of accidents were carried out using the computational fluid dynamics software ANSYS Fluent. The simulation results show that local boiling will not occur in flow channels and the maximum fuel temperature will be lower than the allowable temperature when non-adjacent channels are completely blocked or the maximum blocking rate of adjacent channels is less than 35%. Therefore, the safety operation boundary of JRR-3M reactor under flow channel blockage accident can be determined.