Abstract: Due to the non-monotonic change of the return-stroke velocity(RSV), the return-stroke engineering model under the condition of non-monotonic speed is established by dividing the return-stroke flash channel into piecewise linear segments in this paper. The validity and accuracy of the model are verified. Based on the varied speed return-stroke engineering model, the impact of the non-monotonic change of the RSV on the electric field is analyzed. It is found that the RSV mainly acts on the electric field waveform by affecting the radiation field component, and causes the phenomenon of the secondary peak of the electric field. This paper also analyzes the influence of different speed trends on the electric field waveform and the variation of the electric field at the height of the velocity peak. It is shown that when the height corresponding to the peak of the velocity is constant, the faster the RSV increases, the steeper the rising section of the secondary hump of the electric field is; the slower the velocity decays, the less apparent the sunk depth is after the secondary hump. It is indicated that the non-monotonic trend of the RSV has significant influence on the fine structure of electric field. Establishing the new return-stroke engineering model is a good way for us to enhance the understanding and knowledge of the fine structure of electric field.