Abstract: Magnitude is a measure of the size of an earthquake. In practical work such as seismic activity analysis, people usually think that an earthquake has only one magnitude. Many people will have such doubts: why use multiple magnitudes? Why magnitude can’t be unified? In earthquake monitoring, why magnitudes can’t be converted to each other? Why are different magnitudes of the same earthquake different? How to use multiple magnitudes? This paper describes the principle of magnitude determination and the cleverness of magnitude, and discusses the above 11 problems so that scientific researchers and managers can correctly determine and use magnitude in practical work.