Cross-domain constituency parsing is still an unsolved challenge in computational linguistics since the available multi-domain constituency treebank is limited.We investigate automatic treebank generation by large language models (LLMs) in this paper.The performance of LLMs on constituency parsing is poor, therefore we propose a novel treebank generation method, LLM back generation, which is similar to the reverse process of constituency parsing.LLM back generation takes the incomplete cross-domain constituency tree with only domain keyword leaf nodes as input and fills the missing words to generate the cross-domain constituency treebank.Besides, we also introduce a span-level contrastive learning pretraining strategy to make full use of the LLM back generation treebank for cross-domain constituency parsing.We verify the effectiveness of our LLM back generation treebank coupled with contrastive learning pre-training on five target domains of MCTB.Experimental results show that our approach achieves state-of-theart performance on average results compared with various baselines.