The invention relates to the technical field of ultrasonic wave flow measurement, in particular to a dynamic measurement method for an ultrasonic wave measuring instrument. The dynamic measurement method comprises steps as follows: (A), a sampling time difference deltat i is obtained after the ith time of circuit sampling is finished; (B), a sampling coefficient lambda i corresponding to the ith time is obtained according to the sampling time difference deltat i and following equations; (C), the condition of sudden change of flow is judged according to the (i-1)th sampling coefficient lambda (i-1) and the ith sampling coefficient lambda i, if jumping change happens, the sampling frequency is adjusted and a step E is started, and otherwise, a step D is started; (D), a sampling time difference filtering average value deltat (average) is obtained through averaging, deviation percentage f of the ith sampling is calculated, and the frequency is adjusted according to the f; (E),the volume Vi of the ith sampling is obtained through calculation. The next sampling frequency is adjusted continuously during flow measuring, and the system measurement precision is guaranteed; meanwhile, the sampling frequency is adjusted constantly, so that the energy consumption of the system is in a reasonable range, and compared with continuous high-frequency sampling, system power consumption is greatly reduced.