In the process of soil gradual drought, the growth of typical wetland plant Acorus calamus in dry condition and control condition were investigated. The soil water contents of dry group were about 55% (0^th day), 40% (3^th day), 30% (6^th day), 20% (9^th day), 10% (11^th day) and 5% (12^th day) and average soil water content of the control group was (53.49±0.6)% during the experiment period. Meanwhile, Photosystem Ⅱ (PSⅡ) photochemical efficiency (Fv/Fm), quantum yield (Yield), photochemical quenching (qP), non-photochemical quenching (qN) were also measured by a submersible, pulse-amplitude modulated (PAM) fluorometer: Diving-PAM.The results showed that the decline of soil water content in a short term (soil water content declined from 53.86% to 42.6%, on the 0^th-3^th day) was beneficial to the growth of dry group Acorus calamus. Fv/Fm, Yield, qP were much high than those of the control group (P<0.05), while qN was lower than control group. Compared with control group, average plant height, leaf width and leaf biomass, root biomass of dry group increased by 4.35%, 3.13%, 12.50%, 17.82%, respectively. With soil becoming drought gradually (soil water content declined from 42.6% to 18.02%, on the 3^th-9^th day), the growth of drought group Acorus calamus was restrained gradually and yield and qP value began to decrease. The qN value rose from 0 to 0.403 quickly and was much higher than those of control group (P<0.05). The dry group plant began to suffer from soil drought stress and presented low photo-inhibition phenomenon. High qN value detected in the study demonstrated that they could protect PSⅡ by increasing heat dissipating. At the same time, there was no significant difference of Fv/Fm between drought group and control group, which also indicated that photosynthesis structure of PSⅡ was unspoiled. At the same time,Acorus calamus plant reduced water transpiration and promoted water absorption to protect itself from soil drought pressure by reducing leaf area, leaf water content and root water content. On the 9^th day, the leaf width, leaf water content and root water content of dry group declined to 89.1%, 92.5% and 95.8% of control group respectively.With more serious soil drought degree(soil water content declined from 18.02% to 4.5%, on the 9^th-12^th day), the growth of dry group plant was significantly restrained. The yield and qP value decreased quickly and were much lower than those of the control group. The photosynthetic efficiency decreased significantly. Meanwhile, Fv/Fm value began to be much less than that of the control group and qN value reduced to 0, which indicated that photosynthesis structure of PSⅡ has begun to be destroyed and the capacity of dissipating heat has disappeared. On the 11^th and 12^th day, leaf water content of dry group declined to 75.79% and 68.78% respectively. The small leaf began to wither and decline, followed by the big leaf. The above phenomenon indicated that high plant biomass was weak to keep plant water content in the process of soil gradual drought, 80% of leaf water content was the critical value to maintain Acorus calamus plant survival.