The utility model relates to an automatic water compensation device which consists of a tap water pipe, a float, a float switch and a manual switch, and is characterized in that the float is connected with the float switch through a connecting rod and drives the connecting rod to control the float switch to be turned on or off, the float switch is installed on the tap water pipe, and the manual switch is arranged on the tap water pipe. The float is arranged in a pipe trough which is communicated with a desulphurization tower. The bottom of the pipe trough is communicated with the desulphurization tower through small holes. An overflow port and an overflow pipe are arranged at the upper part of the pipe trough. The level of water in the desulphurization tower can be automatically controlled through the float, the float switch and the manual switch which is turned on. When the level of the water in the desulphurization tower is too high, the water can be lead out through the overflow port and the overflow pipe and thereby the water level is ensured not to be high. Since the pipe trough is connected with the main body of the desulphurization tower through a principle of communicating vessels, the float is ensured to be separated from the desulphurization tower and solution in the desulphurization tower does not corrode the float.