A side load in the over-expanded nozzle was studied by means of a three-dimensional numerical simulation method. The MpCCI software was used to link the FLUENT CFD code whose UDF was applied to the ABAQUS FE code to analyze gas flow and nozzle deformation. The structure parameters and the side load under work conditions from 3MPa inlet total pressure to 7MPa inlet total pressure were obtained. Through calculation, the biggest side load under 4MPa inlet total pressure was obtained. The result analysis show that two types of asymmetric shock physics incur strong side loads: the shock transitions, and shock pulsations across the nozzle lip. Moreover, the results show that the inlet total pressure has potential effect on the nozzle side loads. The fluid-solid coupling numerical simulation supports the accuracy of the method of numerical simulation. The simulation provides the base for further study.