The invention discloses a magnetic nanometer temperature measuring method based on effective relaxation time. The method comprises the following steps that magnetic nanometer particles are placed in an object area to be detected inside a nontransparent article; the magnetic nanometer particles in the object area to be detected are subjected to tissue heating with a radio frequency magnetic field, and meanwhile magnetizing response information of the magnetic nanometer particles under the excitation of the radio frequency magnetic field is obtained with a coil of a hollow structure; the amplitude value of any harmonic signal in magnetizing response signals of the magnetic nanometer particles is extracted; effective relaxation time serves as an intermediate variable, and the relation between the amplitude values of harmonic signals and the absolute temperature is established with the harmonic information to solve the absolute temperature. According to the method, a mathematical model between the magnetizing response harmonic amplitude value information of the magnetic nanometer particles and temperatures is established with the effective relaxation time, the temperature information of the article can be rapidly and accurately obtained, and especially the temperatures are measured while a radio frequency field is heated under the premise that no excitation magnetic field is added in magnetic nanometer thermal therapy.