Background: In current clinical setting, NSCLC patients harboring specific driver mutation were usually treated guiding by prior profiling of the primary tumor when developed to brain metastasis. Some studies have shown that circulating tumor DNA (ctDNA) derived from cerebrospinal fluid (CSF) can reveal unique genomic alterations present in brain malignancies. We assessed CSF as a liquid biopsy media and compared to matched plasma. Methods: We performed capture-based ultra deep sequencing on ctDNA derived from matched CSF, plasma of 40 non-small cell lung cancer (NSCLC) patients with suspected leptomeningeal carcinomatosis (LC) using a panel consisting of 168 genes. Results: Among the 40 suspected LC cases, 35 were confirmed to have LC, ctDNA in CSF from the 5 non-LC cases are all undetectable. Circulating tumor DNA was detected in 93.8% of CSF and 66.7% of plasma. We compared mutation profiles and identified 86 and 46 SNVs from CSF and plasma, respectively, with 42 SNVs overlapping. Furthermore, ctDNA from CSF revealed many copy number variations (CNVs) that were not detected from plasma (189 CNVs vs. 3 CNVs). The average maximum allelic fraction (AF) of CSF ctDNA is significantly higher than in plasma (56.7% vs. 4.4% p < 10^-6). Twenty-eight patients were pre-treated with EGFR-TKIs and developed subsequent resistance. EGFR T790M and MET amplification were detected in 21% and 39% in CSF, respectively, showing a unique resistance profile among leptomeningeal metastases patients compared to the general population. Interestingly, 60% of CSF samples harbor TP53 loss of heterozygosity, only 11% of which were detected in the matched plasma samples. Such heterogeneity may reflect unique biological themes for brain metastatic tumor sub-clones. Furthermore, 26 patients received molecular targeted therapy based on the results from CSF, and 23 reported alleviation of symptoms at subsequent evaluations. Conclusions: Collectively, our data reveal that ctDNA derived from CSF provides a unique and more comprehensive characterization of genomic alterations of leptomeningeal carcinomatosis than plasma, supporting the importance of CSF as a liquid biopsy media.