Writing is widely regarded as one of the most important parts in the field of second language learning and teaching. The main assessment elements of L2 writing can be divided into accuracy, fluency and complexity. The evaluation of writing, accuracy, fluency and complexity can not only measure the writing achievement of L2 writers, but also reflect their writing ability. Based on the complexity of writing, this study adopts the dynamic system theory to investigate the multidimensional development of written English complexity. Three sophomores of high-level non-English majors in a university are selected as subjects to track the development and interaction patterns of eight linguistic complexity indices in 21 essays over three semesters at three levels: lexical construction, sentence construction and phrase construction. The results show that the change and development of subjects in this respect are not linear, but there are peaks, troughs, progress and regression. The development of written language has obvious dynamic and variability.