The invention relates to an ultrasonic tooth cleaning 
machine with automatic switch, wherein an electric conductor which is connected with a main frame contacts 
human body; when a working tip contacts tooth, a main frame circuit, a conducting wire and the 
human body tissue between the working tip or a 
root canal broach and the electric conductor form into a loop impedance. Operating state of main frame is controlled through detecting loop impedance or relative value of the loop impedance in response to certain current 
signal: when the main frame is in tooth cleaning state, the main frame is started to work once the working tip contacts tooth, while the working tip moves away from tooth, the main frame stops working; when the main frame is in 
root canal measuring state, the working tip is replaced by the 
root canal broach to prepare for root canal expansion; meanwhile, measuring and display of the length of 
tooth root canal and working status control of the main frame are completed through detecting loop impedance or relative value of the loop impedance in response to certain current 
signal. In addition, a denture palate rack is arranged and is provided with a spittle suction 
pipe and a light-emitting 
diode (LED), wherein effective illumination of 
oral cavity is realized by the LED, while flushing fluid inside 
oral cavity is discharged through the spittle suction 
pipe.