The invention discloses a method for detecting under-rail equivalent bearing stiffness on the basis of transverse vibration characteristics of a steel rail. The method comprises the following steps: measuring characteristic parameters of the steel rail; determining a relation function between transverse four-order resonant frequency and the under-rail equivalent bearing stiffness of the steel rail; measuring the transverse four-order resonant frequency of the steel rail; determining the under-rail equivalent bearing stiffness according to the relation function and the measured transverse four-order resonant frequency. The method has the following advantages: the method is a nondestructive testing method, and stability of a track structure cannot be affected in the detection process; the detection method can be applied to long-term monitoring of the under-rail equivalent bearing stiffness of a continuously welded rail, and stiffness change of railway ballast is judged on the basis of change of the transverse four-order resonant frequency in different detection stages; the method has the advantages of being simple to operate, time-saving and higher in reliability.