The invention discloses an ultrasonic detection method for base table rotational frequency of the mechanical heat meter. The inside of the base table of the mechanical heat meter is installed with a measurement disk which is immersed into water together with two ultrasonic transducers. The two ultrasonic transducers are arranged on the upper part of the measurement disk and present a V shape. One of the ultrasonic transmitting transducers continuously emits ultrasonic to the measurement disk. Half of the measurement is coated with sound-absorbing material, and the other half is coated with reflection material. The ultrasonic signal reflected by the measurement disk can be received by the other ultrasonic transmitting transducer. The amplitude of the ultrasonic receiving signal reflected by the reflection material is large. But the amplitude of the ultrasonic receiving signal reflected by the sound-absorbing material is small. The base table rotational frequency can be judged after the change of the ultrasonic signal reflected by the disk is detected by the signal processing circuit, so the flow quantity of the heat meter can be calculated. The method avoids the defects that the traditional detection method is easily influenced by the rust in water quality or the signal processing is easily interfered by the electromagnetic. The back end processing circuit is simple.