Spectrum is a kind of optical information that represents the properties of the material. By using spectral imager, the spectral image of the objects can be obtained in the field of view. At present, mature spectral imaging techniques require multiple snapshot to get a complete spectral image data cube. The time resolution of the corresponding system is relatively low, which is not suitable for the spectral acquisition of dynamic target, and the acquisition of dynamic target spectrum requires a snapshot imaging technique. Snapshots of spectral imaging has great advantage in dynamic target spectral imaging, the coded aperture snapshot spectral imaging (CASSI) technology is one method that integrates a compressive perceptual computation methodology into the spectral imaging procedure and the data cube reconstruction process. In the process of imaging, the data compression is completed, and CASSI has the advantage of high throughput. It is possible to reconstruct the target spectral data cube and realize the snapshot imaging by using the single exposure data, which makes it possible to monitor the dynamic target. But the targets information is difficult to prove the sparse hypothesis of this method, leading to a large reconstruction error, which is unfavorable to the monitoring of dynamic small targets. On the basis of CASSI, a new method of double dispersion channel coding aperture spectral imaging system is presented, which is used to obtain the spectral data of dynamic small targets in uniform background. The system consists of two channels, each containing a spectrometer whose dispersion directions are perpendicular to each other, and shares a front-end telescope system and coded aperture. Because the dispersion directions of the two channels are perpendicular to each other, the position of the small target and the corresponding coding can be separated from the background for spectral data reconstruction. So this new system can observe small dynamic targets in uniform background area in real time. Assuming that the radiation characteristic of the background changes little before and after the target appears in the field of view, the background spectrum can be calculated using the data before the target appears. And the target spectrum can be recovered through specific algorithm by utilizing the separated data and background spectrum. An imaging process mathematical model is established, and the reconstruction method has been tested. The target was made to randomly appear in a different position, and the PSNR probability distribution of the reconstructed spectrum was speculated. After adjusting the target size, the influence of target size on the reconstruction accuracy was analyzed. At last, the results were compared with the results of TwIST method reconstruction of coded aperture imaging system. Results show that this method improves the precision of target information recovery accuracy, and greatly reduces the computational complexity to realize real-time monitoring for moving targets when target with less than 5×5 pixels in homogeneous background.