The invention provides a method and system for calculating the length of a car based on a geomagnetic sensor. The method comprises the steps of interpolating two voltage waveforms of each car to obtain an interpolation waveform of each car after acquiring the two voltage waveforms of each car, carrying out band-pass filtering on the interpolation waveform of each car in multiple frequency bands and obtaining a band-pass waveform of each car in each frequency band, obtaining a sampling point number of the band-pass waveform of each car in each frequency band and calculating the length of each car according to the sampling point number of each car, dividing the intervals of multiple cars in each frequency band according to the length of each car and calculating the error rate of interval division in each frequency band, and selecting a frequency band with a smallest error rate as a filtering frequency band of band-pass filtering and calculating the length of each car according to the filtering frequency band. According to the method and the system provided by the invention, the stability, reliability and the accuracy of the division of a vehicle length can be improved, and the methodand system can be applied to each car at the same time.