In this paper, we first synthesized ladderlike poly(vinylsilsesquioxane) (LPVS) by stepwise coupling polymerization on the basis of amido H-bonding self-assembling template from vinyltrichlorosilane (VTCS) monomers. We made the substitutional vinyl of LPVS polymerized at one same side (LPVS-SP), then put it in the vacuum conditions and let it carbonized at 500, 600, 700, 800 and 900°C, respectively to observe its thermal behavior. In our experiment, 700°C can be regarded as a distinct turning point of temperature from weight loss to inter-molecules thermal crosslinking, which can be observed by X-ray diffraction (XRD) measurements and Thermogravimetric analysis (TGA). At the same time, SEM and TEM photos of the carbonization products give very rigidity topography at micron scale, which are the auxiliary evidence for our conclusion. We also present one reasonable thermal carbonization and crosslinking mechanism of LPVS-SP at high temperature in vacuum by remaining products weight analysis.