The invention discloses a method for quickly determining downhole transmitting power of a multi-frequency nuclear magnetic resonance logger. The method includes: (1), determining a pulse magnetic field intensity maximum B1max under a calibration transmitting power which each operating frequency corresponds to; (2), carrying out a first test according to each frequency calibration transmitting power value GAcal; collecting a pulse magnetic field intensity measured value B1 and a formation temperature T for each frequency transmission; (3), calculating a pulse magnetic field intensity measured value B1mod; (4), judging whether an error between the pulse magnetic field intensity measured value B1mod collected in real time and a workshop calibration pulse magnetic field intensity maximum B1Max meets requirements or not; (5), according to the pulse magnetic field intensity measured value B1mod of a formation collected in real time, calculating a power value GAnext of each frequency until the requirements are met. The problems that manual use of an existing logger to determine downhole transmission power takes long time, determination is inaccurate and logging quality is low are solved. Formation information is fed back in real time, and the transmitting power is determined quickly. The method is easy to implement and allows automatic accurate finding.