With unique working principle and spectral characteristic, the long wave infrared (LWIR) interferometric spectral imaging is a popular technology with wide application in many fields. In order to miniaturize and light the instrument, a new method of LWIR spectral imaging system based on a variable gap Fabry-Perot (F-P) interferometer was researched. With the system working principle analyzed, theoretically, it was researched that how to make certain the primary parameters, such as, the reflectivity of the two interferometric cavity surface and the wedge angle of interferometric cavity. A prototype was developed and good experimental results of blackbody and polypropylene film were obtained. The research shows that besides high throughput and high spectral resolution, the advantage of miniaturization is also simultaneously achieved in this method.