The Production-oriented Approach (POA) has been developed over the past decade to improve university English instruction in China, and has proven effective. Empirical studies investigating its long-term effects on the quality of university students’ English writing are still under-represented, however. To fill the research gap, we conducted a 16-week experiment, exploring its effects on the syntactic complexity of six Chinese university students’ English writing. The results are as follows: (1) Generally, the POA exerted a positive impact on the syntactic complexity of the students’ writings. (2) In terms of proficiency level, for the higher-level learners, although there were no significant differences among the pretest, posttest, and delayed posttest, there was an improvement from the pretest to delayed posttest in the mean length of T-unit (MLT) and from the pretest to posttest in the complex nominal per T-unit (CN/T), demonstrating the effectiveness of the POA. For the intermediate level learners, there were no significant differences in both the MLT and CN/T, whereas the continuous improvement from the pretest to delayed posttest revealed the POA’s lasting impact on the syntactic complexity. For the lower-level learners, the values of the CN/T showed a significant difference between the pretest and delayed posttest, indicating that the POA had a significant effect on the syntactic complexity of their written language. It is hoped that this study can shed light on an effective approach to teaching English as a foreign language at the tertiary level.