Disorders of consciousness（DOC）, commonly known as “vegetative state”, is one of the most difficult medical problems in the world. At present, there is no exact and effective means for the detection of consciousness in clinical environment. Functional near-infrared spectroscopy（fNIRS）is a promising non-invasive optical neuroimaging technology with significant potential in the field of disorders of consciousness. In this study, the residual consciousness of patients with DOC is detected using mental arithmetic tasks based on fNIRS. The results show that the healthy control group exhibited significant task-evoked hemodynamic responses in the prefrontal cortex. Reliable hemodynamic responses similar to those of healthy control group can also be detected in some of the patients with DOC. At the group level, significant differences in hemodynamic responses are observed between patients with the unresponsiveness wakefulness syndrome（UWS）and minimally conscious state（MCS）during mental arithmetic tasks. The results show that fNIRS combined with mental arithmetic task may serve as a viable neuroimaging approach for detecting residual consciousness in patients with DOC.