Device and method for online detection of multi-component trace gas in negative pressure state

The invention provides a device and a method for online detection of multi-component trace gas in a negative pressure state, to-be-detected gas is introduced into a ring-down cavity, and a laser control center controls laser signals emitted by a tunable laser with wavelengths of lambda1, lambda2 and lambda3; an optical switch controls a laser signal with one of wavelength lambda1, wavelength lambda2 and wavelength lambda3 to be divided into two laser signals with the energy ratio of 99:1 through the optical beam splitter, the laser signal with 99% of energy is coupled into the optical ring-down cavity through the optical isolator, the laser signal with 1% of energy is transmitted into the calibration system, and is in one-to-one correspondence according to the wavelength, a photoelectric converter captures calibration information and transmits the calibration information to the central data processing and control center, the photoelectric detector captures laser signals transmitted from the light ring-down ca