A method for determining plane stresses on an in-service steel structure member based on phase spectrum of ultrasonic transverse wave, including: calibrating stress-spectrum parameters k and c of a replica of the in-service steel structure member; determining a first response frequency of a phase difference and a maximum value of a derivative function of the phase difference of an ultrasonic transverse wave echo of the in-service steel structure member, and obtaining a polarization angle of ultrasonic transverse wave components generated by a birefringence effect; solving a plane normal stress difference and a plane shear stress inside the in-service steel structure member; and separating normal stresses by a shear difference method to obtain three independent plane stress components.