An automatic hydraulically pushed swimming pool bottom cleaner comprising: an underwater cleaner and a control power supply, which are connected together by a cable, wherein: the underwater cleaner comprises a housing cover part, a housing body part, a filter, a motor shaft assembly, a hydraulically-driven impeller combination, a water inlet/outlet flow passage, wheels and brackets, an isolating hood and the cable; characterized in that the motor shaft assembly is a double extension shaft motor assembly, with a shaft sleeve fixed in the root of a respective outrigger shaft thereof at both ends thereof; an impeller being sleeved on the end portion of each respective outrigger shaft, being axially slideable and rotatable therewith; wherein the impeller combination comprises a hydraulically-driven jaw-type clutch arrangement comprised by two meshing end surfaces with sharp teeth on the respective impeller hub and the respective shaft sleeve wherein each respective impeller is both engageable and disengageable with the shaft sleeve; wherein the housing cover part, the housing body part, the double extension shaft motor assembly, the two hydraulically-driven jaw-type clutch impeller combinations, the water inlet/outlet flow passages and the cable constitute an axial-flow pump with double water injection nozzles; such that along with the changing of the rotating direction of the motor, the impellers and the shaft sleeves are reliably engaged and disengaged under the action of hydraulic power to change the working state of the impeller arrangement. an underwater cleaner and a control power supply, which are connected together by a cable, wherein: the underwater cleaner comprises a housing cover part, a housing body part, a filter, a motor shaft assembly, a hydraulically-driven impeller combination, a water inlet/outlet flow passage, wheels and brackets, an isolating hood and the cable; characterized in that the motor shaft assembly is a double extension shaft motor assembly, with a shaft sleeve fixed in the root of a respective outrigger shaft thereof at both ends thereof; an impeller being sleeved on the end portion of each respective outrigger shaft, being axially slideable and rotatable therewith; wherein the impeller combination comprises a hydraulically-driven jaw-type clutch arrangement comprised by two meshing end surfaces with sharp teeth on the respective impeller hub and the respective shaft sleeve wherein each respective impeller is both engageable and disengageable with the shaft sleeve; wherein the housing cover part, the housing body part, the double extension shaft motor assembly, the two hydraulically-driven jaw-type clutch impeller combinations, the water inlet/outlet flow passages and the cable constitute an axial-flow pump with double water injection nozzles; such that along with the changing of the rotating direction of the motor, the impellers and the shaft sleeves are reliably engaged and disengaged under the action of hydraulic power to change the working state of the impeller arrangement. the underwater cleaner comprises a housing cover part, a housing body part, a filter, a motor shaft assembly, a hydraulically-driven impeller combination, a water inlet/outlet flow passage, wheels and brackets, an isolating hood and the cable; characterized in that the motor shaft assembly is a double extension shaft motor assembly, with a shaft sleeve fixed in the root of a respective outrigger shaft thereof at both ends thereof; an impeller being sleeved on the end portion of each respective outrigger shaft, being axially slideable and rotatable therewith; wherein the impeller combination comprises a hydraulically-driven jaw-type clutch arrangement comprised by two meshing end surfaces with sharp teeth on the respective impeller hub and the respective shaft sleeve wherein each respective impeller is both engageable and disengageable with the shaft sleeve; wherein the housing cover part, the housing body part, the double extension shaft motor assembly, the two hydraulically-driven jaw-type clutch impeller combinations, the water inlet/outlet flow passages and the cable constitute an axial-flow pump with double water injection nozzles; such that along with the changing of the rotating direction of the motor, the impellers and the shaft sleeves are reliably engaged and disengaged under the action of hydraulic power to change the working state of the impeller arrangement.