In order to resolve the problem of large-angle detection of AC magnet o-optical modulation,a method based on the principles of spectrum analysis is studied.Firstly,this paper establishes the d etection model,then analyzes the detection process of large angle of AC magneto-optical modulation by the spectrum structure and t he spectrum amplitude,and achieves the conclusion that the angle detected by this method is not subject to angle periodic impact; Secondly,it uses the method of relative amplitude of spectrum component to detect the angle in a definite ra nge,that is when a spectrum component amplitude is divided by the adjacent spectrum component amplitude,the value is linear in a c ertain angle range,so it can be used in angle detection when the angle range is known.It also analyzes the detection sensitiv ity and relative error of the method,and it can be used to provide a reference for practical applications.At the same time,the pa per also adopts the signal processing technique of difference divided by sum to eliminate the error factors,such as the unstable po wer of the light source and the sample absorption.The technique will have good potential applications in related fields.