The invention discloses a method for measuring an insertion loss/ transmission coefficient of an underwater sound passive material based on time reversal focusing. The method comprises the following steps of: 1) generating time reversal transmitting signals; 2) acquiring direct signals in the absence of a testing sample, transmitting the time reversal signals generated in 1) by adopting a transmitting-receiving transducer, and recording the direct signals pi in the absence of the testing sample by adopting a hydrophone (generally a transmitting-receiving combined transducer is used as the hydrophone) placed nearby the testing sample; 3) acquiring transmission signals in the presence of the testing sample: putting a passive material sample to be detected at a preset position, transmitting the same time reversal transmitting signals as those in the 2) again by adopting the transmitting-receiving transducer, and recording the signals pt in the presence of the testing sample by the hydrophone; and 4) calculating insertion loss: acquiring the required test sample parameter calculation result by using a transmitting coefficient/ insertion loss calculation formula. According to the method for measuring the transmitting coefficient/insertion loss of medium and low frequency bands, errors can be effectively reduced and the precision is improved; therefore, the method is suitable for full-freqeuncy-band measurement.