To determine the performance of the ratio of Mycobacterium tuberculosis-specific antigen (TBAg) to phytohaemagglutinin (PHA), (TBAg/PHA ratio), in T-SPOT.TB assay for early diagnosis of tuberculous meningitis (TBM). Methods: A total of 148 patients with suspected TBM were prospectively recruited. Cerebrospinal fluid (CSF) samples were obtained for routine, biochemical, and microbiological tests within 24 hours of admission. Peripheral venous blood samples were also collected for T-SPOT.TB assay. Results: 131 out of 148 (88.5% ) patients had a final definitive diagnosis, with 55 cases being diagnosed TBM group and 76 cases being diagnosed non-TBM group. The positivity rate of T-SPOT.TB assay in the TBM group was significantly higher than that in the non-TBM group (P<0.05). Using T-SPOT.TB assay, the sensitivity and specificity in diagnosing TBM were 81.8% and 76.3%, respectively. Using 0.07 as the cutoff value of TBAg/PHA ratio, the sensitivity, specificity, and area under the ROC curve (AUC) in the diagnosis of TBM were 78.2%，92.1%，0.871, respectively. Conclusion: Calculation of the TBAg/PHA ratio showed important clinical value for early diagnosis of TBM based on its increased specificity.