Abstract: High precision sandwich reflector panels are made of aluminum face sheet and honeycomb core treated by the gap stress releasing process. In order to study the impact of the gap on sandwich structure stiffness loss, a relationship between the gap influence coefficient considering the influence of the area and distribution of gap on the rigidity and stiffness loss coefficient was established with statistical analysis of tests data. A stiffness loss assessment method based on fracture features of sandwich structure was put forward. The experimental analysis results show that gap will cause stiffness loss, which changes slowly after reaching a certain level and the fracture features influence on the stiffness can be limited into 40%. According to the sandwich structure stiffness equivalent principles and the stiffness loss assessment method, the face sheet with gaps can be replaced by the continuous sheet with decreased thickness during the numerical analysis. It has general application value to simulate the sandwich structure with fracture features.