Disclosed are a method and system for brain activity detection. The method is: performing multi-channel synchronous collections of brain electrical signals and cerebral cortex blood oxygen signals simultaneously, and ensuring synchronicity of the collected signals among channels, and collecting said brain electrical signals and said cerebral cortex blood oxygen signals of all locations at the same time. The system comprises: a functional near-infrared light source emission module (2) which employs the frequency division multiplexing technique, wherein the light source is modulated by carrier of different frequencies, said signal is accessed from the multi-functional joint collection helmet (1) through a transmission optical fiber to irradiate the scalp, and after being scattered and absorbed by the brain, the attenuated light signal is processed by the functional near-infrared detection module (3)； the functional near-infrared detection module (3) is used for detecting weak optical signals of the scalp； the brain electricity detection module (4) is used for detecting weak electrical signals of the scalp； the central control unit (5) is used for synchronizing and fusing data flows, sending control commands to each functional module, and uploading data to the host computer (6). The method and system can control the interference to be the minimum and have good time scale consistency.
