The voltage and current in modern power systems are no longer ideal steady-state sine waves, and have complex dynamic characteristics. It has?attracted some attention on how to accurately measure electric energy under dynamic conditions. The paper, based on the S transform principle, proposed a dynamic electric energy metering algorithm, which divides electric energy into four components, i.e. DC electric energy, fundamental wave electric energy, harmonic electric energy and inter-harmonic electric energy. To verify the proposed algorithm, various dynamic voltage and current waveforms are used as test signals, and the performance and errors of the electric energy metering under steady and non-steady states are analyzed. The obtained results verify that, compared with two conventional algorithms, the proposed algorithm has a better accuracy for measuring electric energy under dynamic conditions.