From the perspective of complexity theory and based on Robinson's Cognition Hypothesis and the Triadic Componential Framework, this paper investigated the effect of manipulating task complexity along resource-dispersing dimension on L2 written performance. The results showed that: 1) Significant interactive effects were found between the two variables (i.e. task structure and planning). 2) Without planning, the accuracy and fluency of written output in tasks without structural constraint were significantly higher, while the syntactic complexity was lower. 3) Planning had no significant effect on accuracy. In macro-structure given task, planning promoted fluency and lexical complexity, but did not affect syntactic complexity. The result only partially supports the Cognition Hypothesis. Combined with previous research, it can be found that written output in tasks is nonlinear, multidimensional and self-adaptive. Researchers and teachers are suggested to fully consider task characteristics and individual differences, and not to take task complexity as the sole criterion when designing writing tasks.