In view of the voltage fluctuation problem of the bidirectional AC/DC converter in islanding AC and DC hybrid microgrid during external disturbance, a disturbance observer of the bus voltage of the bidirectional AC/DC converter is designed in this paper. With this observer, the system disturbance under the circumstance of external impact such as the abrupt change of distributed generation output and load power can be tracked quickly without additional voltage or current sensors, which ensures the plug and play function of the distributed generation and load in the hybrid microgrid. A voltage fluctuation control strategy of the bidirectional AC/DC converter based on the disturbance observer is proposed further to effectively suppress transient voltage fluctuation and impact, which can improve the dynamic response performance and robust stability of the islanding hybrid microgrid under different external disturbance. The simulation model of the islanding AC and DC hybrid microgrid is established in PSCAD/EMTDC to verify the validity and effectiveness of the proposed method with several transient process cases. This work is supported by National Key Research and Development Program of China (No. 2017YFB0903601), National Natural Science Foundation of China (No. 51777088), and Science and Technology Project of State Grid Hubei Electric Power Company Limited Economic Research Institute (No. SGHBJY00PSWT1700080).