Fly ash contents can be considered as a basis for optimal and stable boiler combustion control and fly ash quality control in power plants, especially the unburned carbon in fly ash. The real-time and quantitative measurement of the contents in fly ash was studied using a constructed two-stage cyclone measurement system and detected using the laser-induced breakdown spectroscopy (LIBS) technique. The surrounding gas effect, such as the CO2 effect on the unburned carbon content, was studied comprehensively in this paper. The CO2 effect was eliminated using this proposed combination method of a two-stage cyclone measurement system and LIBS with a 1 ns pulse-width laser according to the efficient gas-particle separation and the controlled laser-induced plasma processes of particle flow. The quantitative analysis was improved using the plasma temperature correction method, with the intensity ratio of the emission pair from magnesium as the plasma temperature indicator. The measurement of the unburned carbon content in fly ash with the temperature correction method presented the concordant results analyzed by the chemical analysis method. The feasibility and improved detection ability for the real-time measurement of fly ash contents in power plants are demonstrated.