With applications ranging from the desktop to remote sensing, the long wave infrared (LWIR) interferometric spectral imaging system is always with huge volume and large weight. In order to miniaturize and light the instrument, a new method of LWIR spectral imaging system based on a variable gap Fabry-Perot (FP) interferometer is researched. With the system working principle analyzed, theoretically, it is researched that how to make certain the primary parameter, such as, wedge angle of interferometric cavity, f-number of the imaging lens and the relationship between the wedge angle and the modulation of the interferogram. A prototype is developed and a good experimental result of a uniform radiation source, a monochromatic source, is obtained. The research shows that besides high throughput and high spectral resolution, the advantage of miniaturization is also simultaneously achieved in this method.