Aiming at information leakage of embedded systems, electromagnetic side-channel test technology is researched. A communication system model including electromagnetism-side(EM-side) channel of Embedded systems was established and a EM-side test platform was built. Based on this test platform, Encryption key capture experiments of FPGA-based Advanced Encryption Standard (AES) algorithm chip were completed. The proposed algorithm in this paper improves the EM-side differential analysis method in time-domain effectively by solving the signal synchronization, the higher demand of SNR and increasing the sensitivity of analysis. Experiment results show that test platform and the EM-side test and analysis method in this paper are feasible and effective.