With the development of infrared remote sensing technology, the demand for infrared detector arrays in various aerospace applications has exceeded the current developmental limit of single-module detectors. This problem needs to be solved by optical or mechanical splicing methods. Based on an advanced mechanical splicing technology, this study presents four design points of splicing structure and their influence on detector imaging for the development of an 8-module ultra-long linear splicing infrared detector. The specific design process of the design points and design results of the splicing structure are introduced in detail. The method for testing a splicing structure and a non-contact flatness test method are described; the test results are presented.