Abstract: In this paper a process of mid-α scale South China LLJ and its heavy rain without front system is successfully simulated by MM4 and ECMWF large scale data. Based on the continuous results with high resolution outputed by the model, a great deal of diagnostic analysis can be completed.It uses the experience of other study of LLJ structure,mechanism and its relation with heavy rain for reference,especially for lack of observeddata with high resolution.In this paper only the results of numerical simulation are given.