Fuel cells exhibit high energy conversion efficiencies and do not have polluting by-products, thus having widespread applications in recent years. Due to the complexity of system operating conditions and sensitivity to parameters, the state monitoring of fuel cell systems has become a significant research focus. Electrochemical impedance spectroscopy (EIS) is a method used for the impedance analysis of fuel cells. By mapping the impedance spectrum, it allows for the monitoring of fuel cell status and the identification of internal faults. Traditional EIS detection techniques require an additional external excitation signal source, making real-time detection difficult. To avoid the use of additional equipment for the real-time monitoring of proton exchange membrane fuel cells (PEMFCs), this paper proposes a real-time EIS detection method based on based on the load voltage detection of a DC/DC circuit. By superimposing triangular waves of different frequencies with the reference voltage to introduce excitation, the output voltage and current of a PEMFC are measured and then processed through fast Fourier transform. The resulting amplitudes and phases at each frequency are translated into the corresponding internal impedance, forming the EIS. The simulation results show a good consistency with theoretical calculation and verify the accuracy and feasibility of the proposed method.