Ash is an important component of foliar litter, the concentration of which is closely related to the process of litter decomposition and the concomitant organic matter dynamics. Ash dynamics in decomposing litter may be affected by environmental conditions and decomposition period. However, currently there is little information about the dynamics of ash concentration in decomposing litter. To fill this knowledge gap, we investigated the dynamics of ash mass and concentration in decomposing foliar litter (litterbag method) of four dominant species (Salix paraplesia, Rhododendron lapponicum, Sabina saltuaria, and Larix mastersiana) under different environmental conditions (forest floor, stream, and riparian zone) in an alpine forest in the upper reaches of the Minjiang River. The results indicated that ash mass in decomposing foliar litter generally decreased over the 2-year experiment, regardless of litter species and environmental conditions, but that there was variation in the magnitude of the decrease. In contrast, ash concentration in all litter species showed a general increasing trend (5.86%-72.15%), except for S. paraplesia foliar litter in streams, which decreased approximately 0.99% over the two years. Changes in ash concentration varied significantly among different decomposition periods, and were significantly affected by initial litter quality and environmental factors. Overall, our results are inconsistent with the historical opinion that ash concentration is relatively stable during litter decomposition, which suggests that the method for calculating ash-free mass loss, using the initial ash concentration value based on the assumption that ash concentration is relatively stable during litter decomposition, is incorrect. The results of this study can provide the basis for a better understanding of ash dynamics during litter decomposition, and the correct method for calculating litter mass loss.