To satisfy the online detection requirements of the ultralow emission of particulate matter in a coal-fired power plant, this paper proposes the online detection method by the light intensity modulation technique based on the laser-scattering method. In addition, the wavelength modulation technique is combined to propose the synchronous measurement method of gas and particle concentrations. Meanwhile, a mathematical model of the scattered-light signal under wavelength modulation is built. The theoretical analysis and simulation results show that the gas concentration is proportional to the height of the normalized second harmonic amplitude peak and that the particulate matter concentration is proportional to the amplitude of the first harmonic of the scattered-light signal from which the gas absorption has been subtracted. A 1392-nm near-infrared laser is used to simultaneously measure the particulate matter and water vapor concentrations during mosquito coil combustion. The experimental results show that the deviation between the measured water vapor concentration value and that by the temperature-humidity sensor is less than 3%. Moreover, the characteristic value of particle concentration has a high linear relationship with the measured value by the dust meter and the fitting factor R2 is 0.9973. The synchronous measurement of particulate matter and gas concentrations is successfully confirmed based on the wavelength modulation technique.