Silicone gel insulation has been widely used in power electronic devices, serving as the encap-sulation material. Its dielectric strength under high-frequency voltage determines the reliable operation of the devices. Electrical tree is a typical failure of solid insulating materials, and the electrical tree propagation in silicone gel under bipolar square-wave voltage are investigated in this paper. The results show that electrical trees are pearl-line like, bush-like and bubble-like under varied voltage conditions. After the rapid growth period, the electrical trees stagnate under frequency lower than 20 kHz while develop steadily when the voltage frequency is higher than 25 kHz. The number of tree branch, fractal dimension and accumulated damage increase with the voltage frequency. In comparison, the electrical trees propagate slowly under sinusoidal -wave voltage. The partial discharge and photo-degradation corresponding to the fluorescence are found in main tree channel, which are related to voltage waveform and lead to different electrical tree behavior. The injection and transportation of space charge accelerates the electrical tree propagation, especially at the rising/falling edge of bipolar square-wave field due to polarity reversal. Besides, the amorphous carbon deposited near the needle tip and the bubbles in tree channels filled with hydrogen and carbon monoxide are all related with the electrical tree propagation.