The invention relates to a non-intrusive ultrasonic detection method of mud flow rate in a deepwater drilling riser. The method comprises the steps that 1) an emission sensor is close to the wall of a riser, and a receiving sensor is arranged on an ultrasonic wave propagation path in the direction vertical to the wall of the riser; 2) the actual work signal frequency and the echo signal intermediate frequency shift information of the emission sensor are acquired; 3) difference frequency signals are acquired, and the difference frequency signal frequency is calculated; 4) the difference frequency signal of the theoretical difference frequency signal frequency maximum is rejected; 5) time frequency conversion is carried out on the difference frequency signals to acquire the frequency domain information of amplitude-frequency or power-frequency; 6) difference frequency signals with amplitude value less than 2% of the maximum amplitude value or with power value less than 2% of the maximum power value are removed, and weighted integral is carried out on remaining frequency to acquire a frequency difference result; 7) a curve of the relationship between the mud flow rate and the frequency difference result is acquired; and 8) according to ultrasonic, the mud flow rate is calculated through the frequency difference result of the riser. The non-intrusive ultrasonic detection method provided by the invention can be widely used for the detection of the mud flow rate in the deepwater drilling riser.