The invention provides an automatic paddy field water draining device and belongs to the field of environment-friendly planting of rice. According to the automatic paddy field water draining device, the outer surface of a vertical smooth round pipe of an L-shaped water outlet pipe is sleeved with an annular floating body, a matching face of the annular floating body is matched with the pipe surface of the vertical upper portion of the L-shaped water outlet pipe so that the annular floating body can slide on the upper portion of the water outlet pipe, and the annular floating body can open or close a water draining channel when sliding up and down. The automatic paddy field water draining device is compact and reasonable in structure, convenient to manufacture and use and good in automation degree and achieves a good automatic water draining effect and a good paddy field water level keeping effect, during automatic water draining, people do not need to watch over the automatic paddy field water draining device, and when the water surface on a paddy field is lower than a set irrigation water level, the water draining channel is automatically closed so that the water level can be kept. The automatic paddy field water draining device is a safe and environment-friendly water draining device for paddy fields and is a necessary device for automatic management of the paddy fields.