A method for non-destructive detection of the steel strand stress based on electromagnetic resonance was proposed. This method only needs to connect the two unstressed ends of the steel strands as the inductors to the oscillation circuit, and the stress of the steel strands can be measured by collecting the oscillation frequency of the circuit through a frequency meter. The mathematical model of the stress and the frequency was established, and a 7-steel-strand test system was built to verify the mathematical model. Theoretical analysis and experimental study all show that the oscillation frequency of the LC electromagnetic resonance circuit decreases as the stress of the steel strands increases. This method can be used for the rapid detection and evaluation of the effective prestress of in-service bridges.