The invention discloses an online glutamic acid fermentation detection system and an online detection method using the system, which is used for detecting the concentrations of glucose and glutamic acid on line during glutamic acid fermentation so as to solve the problems of disturbance caused due to time-varying characteristics, relevance and hysteresis of biochemical parameters and inevitable manual interference in a fermentation process of glutamic acid and low reliability in implementing a plurality of mathematical models which are generated with relevance to substrate consumption, bacteria growth and product generation. The online detection method for the concentrations of the glucose and the glutamic acid is finished under automatic control of programs completely; manual sampling is not required; meanwhile, the concentrations of the glucose and the glutamic acid can be simultaneously detected and data can be input to a computer to be saved; the glutamic acid fermentation dynamics is researched according to the concentrations of the glucose and the glutamic acid in different fermentation stages, so that the fermentation process is well controlled.