Pemetrexed is one of the first-line chemotherapeutic agents for non-squamous non-small cell lung cancer (NSCLC). Since 2017, intrathecal pemetrexed has shown good efficacy for patients with leptomeningeal metastases from NSCLC. It has been recommended as the preferred drug for intrathecal chemotherapy by the Chinese Society of Clinical Oncology (CSCO) guidelines. Tyrosine kinase inhibitors (TKIs) play a promising role in treating non-small cell lung cancer patients with epidermal growth factor receptor (EGFR) mutations. An international multi-center clinical study published in 2019 confirmed that double dose of osimertinib showed significant improvement in leptomeningeal metastases from NSCLC with EGFR exon 19 deletion or exon 21 L858R/T790M mutation. It makes TKIs the mainstay of treatment for patients with EGFR-mutant NSCLC with leptomeningeal metastases. However, the choice of treatment after resistance to targeted therapy is a hot topic in clinical practice, with 78% of patients in the study above who responded to double-dose osimertinib still showing progression at the time of follow-up. The purpose of this study was to observe the safety and efficacy of intrathecal pemetrexed for leptomeningeal metastasis from lung adenocarcinoma that progressed after a double dose of a third-generation TKI such as osimertinib.