The invention discloses an on-line metering device and a method for a tight oil surfactant active agent displacement huff-puff experiment. The on-line metering device comprises a separation chamber, and an inflow port and an outflow port are formed in the separation chamber. A screen is arranged in the separation chamber, and the screen divides the interior of the separation chamber into isolated spaces arranged up and down; injecting ports communicating with the isolated spaces are formed in the separation chamber; the outflow port is connected with a crude oil metering device; the injecting ports are used for injecting a demulsifier into the isolated spaces; the screen is used for separating the crude oil and the demulsifier; and after being demulsified by the demulsifier and filtered by the screen, the crude oil enters the crude oil metering device from the outflow port for metering. According to the on-line metering device and the method for the tight oil surfactant active agent displacement huff-puff experiment, the influence of emulsification can be eliminated, the metering precision can be improved, the produced oil can be metered completely, the produced oil can be metered by sections, and repeated and batch operation is easy.