Language acquisition is vital to reveal the nature of human language intelligence and has recently emerged as a promising lens for improving the interpretability of large language models (LLMs). However, due to ethical and practical constraints, many experiments that require controlling language inputs remain infeasible with human learners. This poses challenges for the verifiability and scalability of language acquisition modeling, particularly in Chinese second language acquisition (SLA). While LLMs provide a controllable and reproducible alternative, a systematic benchmark to support phase-wise modeling and assessment is still lacking. To address these issues, we propose HSKBenchmark, the first benchmark for staged modeling and writing assessment of LLMs in Chinese SLA. It spans HSK levels 3 to 6, comprising authentic textbooks with 6.76M tokens, 16K synthetic instruction data, 30 test titles and a linguistically-grounded evaluation system. To simulate human acquisition trajectories, a curriculum-tuning framework is introduced, which trains LLMs in a progression from beginner to advanced materials. In addition, since language production in writing is a key perspective for observing SLA development, we establish the evaluation system including the coverage of level-based grammar items, writing errors, lexical complexity, syntactic complexity, and holistic scoring to probe LLMs in writing. We also develop an HSKAgent fine-tuned on 10K compositions from Chinese second language learners to automate this evaluation system. Extensive experimental results demonstrate that HSKBenchmark not only models Chinese SLA effectively, but also serves as a reliable benchmark for dynamic writing assessment in LLMs. Our fine-tuned LLMs have writing performance on par with advanced human learners and exhibit human-like acquisition characteristics. The HSKBenchmark, HSKAgent, and checkpoints serve as foundational tools and resources, paving the way for future research on language acquisition modeling and LLMs interpretability. Code and data are publicly available at: https://anonymous.4open.science/r/HSKB-5E70.