Many codes allow designers to use the bending moment diagram computed by elastic analysis and modify it by a certain amount of moment redistribution (MR) to account for plastic behaviour of continuous beams. However, several researchers indicated that the MR at the ultimate limit state (u) for some beams deviate significantly from the specified values of various codes. This paper examines the applicability of the provisions on u in ACI 318-19 and Eurocode 2 through numerical investigations and comprehensively explores the influencing factors. The results show that some parameters not considered in those codes influence u to a certain extent, where the ratio of tensile reinforcement ratio at intermediate support to tensile reinforcement ratio at midspan (s1/s2) and load type are crucial parameters to consider. The specific combination of these two parameters may make the codes overestimate or significantly underestimate the u. On the other hand, the yield state of both critical sections is found to have an important influence on the influence degree of each parameter on u. The yield conditions are investigated, and an empirical judgment equation is proposed. In addition, the influence laws of the critical parameters on u have been further proved by theoretical derivation. Finally, due to t is found to have a better linear correlation with βu than xu/d, equations as a function of t for predicting the u of continuous beams under the two loads are proposed, respectively.