The invention discloses an online system for collecting and processing data in a biogas fermentation process. The system consists of a gas measuring and transmitting device, a signal transmitting and receiving device, a data processing device, and a setting and displaying device, wherein the fermentation state in a fermentation tank is detected in real time through the gas measuring and transmitting device mounted on a biogas fermentation tank; information is transmitted to the data processing device through the signal transmitting and receiving device, then calculated and finally displayed on the setting and displaying device. According to the online system, a CH4 transmitter, a CO2 transmitter, an O2 transmitter and a H2S transmitter are mounted on a filtering and gas collecting device to detect changes of gas components in the biogas fermentation process, and parameters in the fermentation process is calculated by a PLC (Programmable Logic Controller), displayed and recorded in an online manner in real time, so that the visualization of changes of the gas components in the whole biogas fermentation process is realized and the theoretical basis is provided for improvement of the fermentation process and the fermentation device. In addition, the system disclosed by the invention is simple in structure, high in control precision and good in transportability.