Researchers have been investigating the effects of different planning conditions on learners' second language (L2) writing performance. However, whether such effects differ regarding specific planning conditions and task complexity has remained underexplored. This study examined the effects of pre-task planning (PTP) versus unpressured online planning (OLP) on L2 writing, particularly in relation to task complexity manipulated in terms of the reasoning demands. Participants were 60 low-intermediate Chinese English as a Foreign Language (EFL) learners who were divided into a PTP group and an unpressured OLP group. Each group completed two writing tasks differing in complexity manipulated as more or less reasoning demands. The participants' texts were examined in terms of lexical and syntactic complexity, accuracy, and fluency. The results revealed that there were no significant differences between PTP and unpressured OLP in lexical and syntactic complexity. However, unpressured OLP benefited accuracy and fluency, and the benefits reached a significant level. In addition, its effects on accuracy were affected by task complexity in that it was much more beneficial to a simple task than to a complex one. For lexical and syntactic complexity and fluency, no such effects were found. The pedagogical implications and limitations of the study were discussed.