Acid rain is an important phenomena in global climate change and it can strongly influence forest ecology. Many studies have shown the impacts of acid rain on root morphology, but the results are very different due to the different species and environmental conditions. Till now, the effects of acid rain on the root morphology of Moso bamboo has not been reported. Moso bamboo(Phyllostachys pubscens), an economically important plant, is often cultivated not only for its delicious shoots and versatile culms, but also as an important biomass resource in southern China. However, with its robust growth and strong rhizomes, Moso bamboo populations have been expanding rapidly into adjacent forests. The ability of Moso bamboo plant to respond to competition through root plasticity may be an important trait for the species, but little information is available on how root systems respond to belowground competition during simulated acid rain. To provide baseline information on the morphological plasticity of the response of Moso bamboo roots during the simulated acid rain we continuously mixed Moso bamboo and broad-leaved forest(transition forest). Three gradients of simulated acid rain treatment including pH 2.5, 4.0 and CK (lake water)were designed to determine the effects of the simulated acid rain on root plasticity in transition forest. This study was conducted in a typical transition zone in the TianMu Mountain Nature Reserve, Zhejiang Province, China, where there is ongoing bamboo expansion. Rhizome and root from transition forest was collected by the soil core method to analyze root morphological parameters. The results showed that(1)T1(pH=4.0)treatment significantly inhibited the root system of broad-leaved trees(P<0.05), resulting in 39.9%, 39.4%, 42.3% and 16.2% decrease in total root length, total surface area, total volume, and special root length, respectively. T2(pH=2.5)treatment significantly inhibited the growth of Moso bamboo and broad-leaved trees(P<0.05).(2)In the vertical direction, the root system biomass of Moso bamboo in 0-10cm soil layer increased from 81.9% of all root biomass in CK to 87.2% in T2. Simultaneously, the root biomass of broad-leaved trees shifted from the upper to lower soil layers. The root system of Moso bamboo managed to place more biomass in upper soil layers when it encountered other trees, allowing it to gain a competitive advantage.(3)In the horizontal direction, the root biomass of Moso bamboo did not significantly decline with an increase in distance from the sample tree, while the root biomasses of broad-leaved trees decreased with the increasing distance from the sample tree. The inhibition of acid rain on root system was independent of horizontal distance.(4)The pH, organic matter, soluble organic carbon, soil respiration rate, available nitrogen, available phosphorous and available potassium had significant effects on root plasticity of Moso bamboo(P<0.05). It can be seen that, under simulated acid rain, the morphological plasticity of Moso bamboo roots is stronger than that of broad-leaved trees, showing an overall trend of accelerating invasion. Moso bamboo would outcompete trees in the belowground environment. It is of great significance to clarify the competitive position of root plasticity under simulated acid rain for forestry management.