The invention provides an automatic speed adjustment energy-saving centrifugal water pump. A sensor (3) is mounted in a water outlet of a pump body by using the characteristic of direct proportion between the rotating speed and the voltage of a brushless direct-current motor, and is electrically connected to a motor control system (18); and after users set an adjusting operation panel (19), the motor control system (18) controls a duty ratio of PWM signals according to signals acquired by the sensor (3), so that the average voltage of multiple coil windings of a stator (15) can be controlled along with change of the medium temperature or the pipe system resistance, the rotating speed of the brushless direct-current motor (1) is adjusted, and the energy consumption is reduced.