Testing core loss under different operating conditions in switched mode power supplycan avoid the unexceptional temperature rise. The test platform for core loss based on power amplifier is simply and feasible. However, the exciting current flows through the output resistance of power amplifier will cause the excitation waveform smaller than the expected. In this paper, we propose the compensation method to reduce the influences caused by the output resistance of power amplifier. The experimental resultsshow the proposed method isuseful.