Based on the fatigue testing machine，this study independently designed bolt loosening test devices under complex loads. The loosening process of bolted joints under complex load had been investigated under many different experimental parameters, and the loosening behavior of bolted joints of different load amplitude and load type under complex load were systematically investigated. Combining the microscopic analysis using optical microscope and obtaining the real-time residual clamping force of bolt connection and the relative rotation angle of bolt and nut, this paper revealed loose mechanism of composite bolt connection. The main conclusions obtained in this study are as follows: When the load amplitude is larger, the residual clamping force of bolt connection decreases more, and the connection structure is more prone to relaxation. The increase of axial amplitude leads to the aggravation of the damage around the hole of the composite plate; The increase of transverse amplitude leads to the increase of wear of composite board surface; The load type of complex load obviously affects the relaxation behavior of the connection structure; Compared with static tension load, vibration load has greater influence on the residual clamping force of bolt connection.