The invention discloses an adjustable biochemical tank and an adjustable AAO process for water treatment. The adjustable biochemical tank is characterized in that: the two sides of a water intake pipe are respectively provided with an oxygen deficiency pond, an anaerobic pond and an aerobic pond, wherein, the oxygen deficiency pond and the anaerobic pond are vertically arranged between the aerobic pond and the water intake pipe side by side, and the oxygen deficiency pond and the anaerobic pond are next to the water intake pipe; the water intake pipe is communicated with the oxygen deficiencyponds and the anaerobic ponds which are arranged on the two sides through a weir gate I and a weir gate II, the oxygen deficiency pond is communicated with the anaerobic pond through a hole, and the anaerobic pond and the oxygen deficiency pond are respectively communicated with the aerobic pond through a gate I and a gate II; and between the aerobic pond and the oxygen deficiency pond is provided with a digestive juice reflux pump, and blenders are respectively arranged in anaerobic pond and the oxygen deficiency pond. According to the invention, the designed structure is reasonable and compact, the proper process can be selected according to the water quality, the effects of dephosphorization and denitrification, the efficiency of water treatment and utilization rate of the device can be improved, the cost can be saved, and the occupied area can be reduced.