Abstract: The complex forming process of the composite material leads to low manufacturing precision. Assembly gap will be created between the mating surfaces due to the deviation of composite parts. Gap-filling needs to be taken when the thickness of gap exceeds a certain extent. Based on a simplified assembly model, the effect of liquid shim on strain and stress of composite assembly structure with bolt pre-tightening force was studied by means of experiment and finite element analysis. The surface strain under two conditions was compared to analyze the deformation of composite parts, including forced assembly situation and gap-filling situation with liquid shim. The strength ratio was calculated by extracting stress component of element integral points. And the effect of liquid shim on stress of composite assembly structure based on strength ratio was obtained. The primary conclusions are as follows:The area on composite near the edge of the assembly gap is affected by the assembly gap more greatly during forced assembly, while the impact of gap on middle section is relatively small because assembly gap disappears in this area under the clamping force. Liquid shim is helpful to improve the strain state of danger zone near the edge of the assembly gap, but the strain of the compression zone contacting with bolt head which located in middle section of composite is increased due to introduction of liquid shim. Overall, liquid shim makes the strain distribution more uniform. When the assembly gap thickness is more than 0.7 mm, the effect of liquid shim that enhances the strength ratio of the component shows obvious downward trend with the increase of assembly gap thickness.