The microenvironment of litter affects its decomposition to a great extent. However, how the microenvironment alters in Robinia pseudocacia plantations with increasing stand age, which are widely distributed in the Loess Plateau, and how these alterations affect litter decomposition in plantations remain unclear. To answer these issues, the typical recalcitrant and labile litters of Pinus tabuliformis (Pt) and Trifolium repens (Tr) were chosen for this study. The tested litters were fixed on the floor of R. pseudocacia plantations with stand ages of 10, 20, 33 and 43 years for 592-day decomposition. The tendency of alterations in litter decomposition and nutrients release, and soil physical (temperature and moisture), chemical (content of available N and P, and pH) and biological properties (community composition and structure of fungi and bacteria, and enzymatic activities) at given decomposition stages with the increasing stand age were detected. The relations between litter decomposition rates and soil micro-environmental properties were analyzed. The results indicated that: (1) Pt litter exhibited decrease-increase decomposition rates with the increasing stand age, its C release rate at all stages of decomposition exhibited decrease-increase with the increasing stand age as well, while its N and P release rates only exhibited the same pattern at the late decomposition stage. Tr litter exhibited the increasing decomposition rates with increasing stand age (P<0.05), while its C, N and P release rate at all stages of decomposition the same pattern as well. (2) Generally, the surface temperature of soil in plantations exhibited a decrease-increased tendency. The soil moisture, available P content and pH decreased with the increasing stand age, while the available N content exhibited an opposite trend (P<0.05). (3) The increase in stand age significantly altered the soil fungal and bacterial community structure, especially, it caused significant successions of fungal genus. Comprehensively considering the activity of nine litter-decomposition associated enzymes, both the total enzymatic activity and lignocellulases activity exhibited a decrease-increase trend in the 108^th day of litter decomposition, while they significantly increased with increasing stand age in the 389^th and 592^nd days (P<0.05). (4) The decomposition rate of Pt litter was positively related with soil total enzymatic activity, fungal community structure and ammonium N content, while that of Tr was positively related with soil total enzymatic activity and lignocellulases activity, fungal and bacterial communities structure, temperature and ammonium N content, and negatively related with soil moisture and pH (P<0.05). In conclusion, the alterations in the physical, chemical and biological microenvironment in R. pseudocacia plantation caused by increasing stand age are generally in favor of the litter decomposition processes.