Chemical and antioxidant responses of the water-deficit tolerant Chinese liverwort, Plagiochasma appendiculatum, to water-deficit stress were investigated in this study. The results showed that water-deficit stress could increase the accumulation of the main bisbibenzyl, riccardin D (RD), and the level of the expression of genes related to production of RD. Activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), soluble protein and malondialdehyde (MDA) content could be used as indicators of the degree of damage to plants under different water-deficit stresses. Among these indicators, SOD played a significant role in defense against water stress when humidity is higher than 40 ± 5% RH (relative humidity), while the activities of POD and CAT were inhibited. However, when humidity is lower than 40 ± 5% RH, POD and CAT activities were accelerated, and if water-deficit reached serious levels (≤ 20 ± 5% RH), POD activity was inhibited, while CAT activity was long-lasting and enhanced; soluble protein and MDA content exhibited the opposite trend. Below 70 ± 5% RH, MDA contents were the highest among all treatments, suggesting that over-wetting is a significant stress for this liverwort. Content of MDA is higher after one week rather than two weeks, suggesting that the liverwort has the ability to eliminate MDA produced by membrane lipid peroxidation. The results of this study provide optimized soil humidities to improve the production of riccardin D in a cultivated liverwort.