Based on the concept of splitting wavenumber difference, this paper clarifies some confusing and incomprehensible problems in electron charge-mass ratio measurement experiment by Zeeman effect, such as the determination of the observed object and its wavenumber difference, the spectrograde overlaping method, etc. According to free spectral range of F-P etalon and the wavenumber difference distribution map of Hg 546.1nm Zeeman splitting spectral, the spectrograde overlapping pattern are deduced. It can improve students'understanding of measurement methods and also provide a certain reference for teaching.