The trace elements emission could result in harmness to the environment and human health during coal combustion. The removal of trace elements before coal combustion is an effective method. The transformation and removal behavior of arsenic in Wujiaping (WJP) coal during sub-critical water treatment was studied in a semi-continuous apparatus. Effects of temperature, pressure, residence time and water flow rate on the arsenic removal were discussed. The occurrence modes of arsenic in WJP raw coal and residuals treated by sub-critical water were also studied by float-sink experiments and sequential chemical extractions. The results show that with the increasing of temperature, pressure, residence time and water flow rate, the arsenic removal in WJP coal also increases; and at the condition of 380 ℃, 15 MPa, 60 min and water flow rate of 0.58 L/h, the arsenic removal is about 62%. Arsenic in WJP coal mainly is associated with sulfates & mono sulfides, disulfide & some organic matters and “insoluble forms”. Sub-critical water treatment can decrease all these arsenic forms in coal and transfer them into the volatile form.