Infrared (IR) dim small target detection in heavy noise has always been a challenging research task. Detection methods based on energy accumulation have achieved effective detection results. However, the general inaccuracy of the target energy distribution model for subpixel motion weakens the performance of these detection methods. The discretization process of a target is described in detail, and the effect of two-dimensional (2D) subpixel motion on the energy spatial distribution of the target is introduced. Based on the analysis of target energy distribution for subpixel motion, a novel energy spatial distribution model of the target for 2D subpixel motion (NESDSM) is proposed. The NESDSM model well describes the energy spatial distribution variation of a subpixel moving target. Furthermore, to verify the accuracy of the model, a target detection method based on energy compensation accumulation (ECA) is proposed. The ECA synchronously enhances the signal noise ratio and suppresses noise effectively. Finally, experimental results show that ECA has better detection performance in comparison with traditional energy accumulation methods. Moreover, by analyzing the performance of ECA in a quantitative manner, ECA shows an average 10.5% improvement in the output SNR in comparison with energy accumulation methods.