The invention discloses an Mg element ratio based water content calibrating type heavy metal content detecting method for leaves. The method comprises the following steps: 1) acquiring leave samples with different heavy metal contents, and preprocessing the samples; 2) detecting the heavy metal contents in the samples through a standard method; 3) acquiring LIBS spectrum lines of different positions of the samples; 4) preprocessing the obtained spectrum signals; 5) extracting LIBS spectrum line peak intensity corresponding to the detected heavy metal element and the Mg element; 6) building a calibration mode by testing the detected heavy metal content as the output and the intensity ratio of the detected heavy metal element to the Mg element as the input; 7) acquiring the spectrum lines of the samples to be detected, and extracting the LIBS spectrum line peak intensity of the heavy metal element and the Mg element from the preprocessed spectrum signal, and inputting into the calibration mode to obtain the heavy metal content. According to the method, the water contents in the samples are calibrated, so that the influence of the water contents in the samples on the heavy metal detection can be removed, and as a result, the detection accuracy is improved.