Abstract: In order to investigate the inter-ply slippage situation and spring back deformation after curing during forming process of structure, a self-designed forming process device for hot diaphragm was used to fabricate the C-shaped structure of thermosetting resin matrix composites firstly. Meanwhile, a three-dimensional numerical simulation model for hot diaphragm forming process was established. The model consisted three composite curing process sub-models, including thermo-chemical model, inter-ply slippage model and curing deformation model. Then, the time-dependent characteristics for properties of composites during curing process were introduced into the simulation model on the basis, and the simulation results were compared with experimental results in reference. Finally, the numerical simulation of hot diaphragm forming process was conducted using the simulation model established, and was contrasted with experiments. The temperature, distribution for degree of cure, inter-ply slippage and curing deformation situation during forming process were mainly investigated. The results obtained prove that the numerical model established has relative high reliability and precision for the prediction of diaphragm forming process, which can provide some references for the optimization of parameters and mold correction of following hot diaphragm forming process.