The disclosure discloses a methane value online real-time monitoring system, which includes a laser tunable system portion, a gas component detection system portion and a data processing system portion. The laser tunable system portion includes a computer, laser drivers, tunable lasers, a laser beam combiner, a collimator, and a beam expander, which are connected in sequence. The gas component detection system portion includes a gas supply system pipeline, a hose, a valve, and a gas chamber. The data processing system portion includes a detector, lock-in amplifiers, an oscilloscope, and a data acquisition card, which are connected in sequence. In a working process of the methane value online real-time monitoring system, gas mass namely the methane value change can be monitored in real time on line by collecting natural gas introduced into a combustion chamber through a gas supply system pipeline of a natural gas engine.