In thermal power plant, the measurement of unburned carbon in fly ash becomes increasingly important because unburned carbon contents are usually important factors for the operation of power plants and it can be an indicator of the combustion conditions Thus, the goal of combustion optimization and efficiency improvement can be achieved. More specifically, the unburned carbon concentration in fly ash reflects the combustion efficiency of the power generator set. The combustion efficiency decreases with increasing of unburned carbon level in fly ash, which means a loss of fuel energy. Owing to the non-contact, fast response, high sensitivity and online measurement features, LIBS was used to measure the quantitative contents in fly ash in this research. But the carbon emission intensity changed with the fluctuant amount of CO2 in flue gas. In this study, dual-channel spectrometer was utilized to analyze the elemental spectra of C, Si, Mg, Fe, Ca, Al and so on, and the neighboring elemental spectra, such as C and Fe, can be recognized in the high resolution channel, which means sufficient fly ash spectrum information can be gained with enough measurement accuracy at the same time. Cyclone was designed and two-stage cyclone system was proposed into the system of LIBS measurement for unburned carbon contents in fly ash. Fly ash particles from the feeder flow through the two-stage cyclone and went into the chamber. The laser shot on the fly ash through the lens. Plasma was acquired and then recorded and analyzed by spectrometer and ICCD camera. The two-stage cyclone system was proven to be detectable for the unburned carbon in fly ash. In addition, two-stage cyclone system can be able to separate and collect fly ash particles from flow gas to reduce the influence of CO2 in fly ash particle flow, thus, exact data can be got to guide the operation of the thermal power plant, which provide more benefits to the engineering applications of LIBS.