Extant research has consistently demonstrated that negative perfectionism is an important driver of procrastination. However, existing studies suffer from several salient limitations, such as an inadequacy in understanding its mediating mechanism, and the lack of an overarching theoretical framework. Accordingly, the present study adopts the ego depletion theory to investigate how and under what conditions negative perfectionism leads to procrastination. Specifically, we propose that fear of failure and ego depletion serially mediate the effect of negative perfectionism on procrastination, and that cognitive flexibility moderates this chain mediating effect. A three-wave survey consisting of 490 participants, in which negative perfectionism was measured in the first survey, fear of failure and ego depletion were measured in the second, and procrastination was measured in the last survey, lends support to all of our hypotheses. Specifically, our results indicate that (a) negative perfectionism influences procrastination through the chain mediating effect of fear of failure and ego depletion; (b) cognitive flexibility moderates the effect of fear of failure on ego depletion in that the effect is weaker when cognitive flexibility is high rather than low; and (c) cognitive flexibility moderates the chain mediating effect of negative perfectionism on procrastination, such that this chain mediation is weaker when cognitive flexibility is high rather than low. Our findings are discussed in terms of the theoretical contribution to reveal the mechanism by which negative perfectionism results in negative effects from the novel perspective of ego depletion.