Abstract: Composite material components may produce gaps during assembly due to their manufacturing errors. It is a basic method to use the liquid shim to fill the gaps. The tensile experiment was designed for composite single-lap bolted joint. An improved three-dimensional failure criterion and the corresponding degradation of the material were chosen in the finite element analysis(FEA) modeling. On this basis, the influences of the liquid shim on the mechanical properties of single-lap composite bolted joint were studied, such as strength and stiffness. The damage evolution process around the hole in composite and the state of stress and strain around the hole in shim were also studied. From the experiment and finite element results, it can be concluded that with the increase of liquid shim thickness, the tensile stiffness and peak load of the joints are reduced. It can also make the hole in composite have more serious damage under the same load and decrease the load when the initial damage appears. But the increase of the liquid shim thickness can decrease the peak stress and the plastic strain of the hole in the liquid shim and make their distribution more uniform, which can improve the stress condition around the shim's hole.