A method for online measurement of the levels of moisture in dead fine fuels on the forest floor is proposed based on near-infrared reflectometry. A linear relationship is found between the moisture content of dead fine fuels and the intensity of light absorption at a wavelength of 1450 nm, and this forms the basic principle of measurement for different forest species. An online moisture-monitoring device is designed and developed based on this principle. This uses long-distance wireless data-transmission to adapt to the lack of telecoms service in remote forests. Moisture-measurement experiments were conducted continuously for 24 h in rainless weather during the summer of 2019 in the Harbin Urban Forestry Demonstration Base of Northeast Forestry University using leaves from four different forest species, specifically, larch, dryland willow, silver birch, and walnut. These measurements were verified using simultaneous measurements employing a weighing method. The results from the field experiments agreed well with the weighing results. The uncertainty in the moisture-content measurements using the proposed system was ∼3.4% when the true moisture content was 20.0% or less. This proposed system has the advantages of online measurement and high accuracy. This method for monitoring moisture levels in dead fine fuels is an important advance in terms of improving the accuracy and simultaneity of measurements for prediction of forest-fire-risk ratings and fire behavior.