Abstract: Laser-induced breakdown spectroscopy (LIBS) was used to measure the trace elements in gas, which has great potential in on-line measurement of trace elements. By setting up a LIBS exprimental system and taking the trace elements mercury and iodine as the objects of study, the trace elements in gas were measured and analyzed using LIBS under low-pressure and different laser wavelength conditions. Results show that the detection capability of LIBS can be significantly improved by lowering the pressure; at different pressures, the detection capability with laser of wavelength 532 nm for trace elements in gas is stronger than that with laser of wavelength 1 064 nm.