The invention discloses an alga solution dewatering device and method based on a dead end positive osmosis technology. The dewatering device comprises an osmosis device, a stirring device and a suction liquid circulation system, wherein the osmosis device comprises a positive osmosis membrane, an upper box body and a lower box body; the upper box body and the lower box body are mutually connectedthrough screw bolts; an alga solution storage groove is formed inside the upper box body in a vertically penetrating way; the suction liquid circulation groove is arranged on the upper surface of thelower box body; a sealing ring is arranged between the upper box body and the lower box body; the positive osmosis membrane is clamped between the upper box body and the lower box body; the stirring device comprises a driving motor, a connecting rod and a stirring paddle; the output shaft of the stirring motor is connected with one end of the connecting rod; the other end of the connecting rod isconnected with the stirring paddle positioned at the inner bottom of the alga solution storage groove; the suction liquid circulation system comprises a peristaltic pump, a connecting pipeline and a suction liquid storage device. The alga dewatering method using the device has the advantage that the alga solution can be concentrated at low energy consumption to obtain filter cake layers with low moisture content.