Disclosed is a structural-stability demonstration instrument including a demonstration platform, a support block and a group of assembled blocks with different gravities. The bottom part of the support block is provided with a group of support feet. The upper ends of the support feet are rotatable connecting parts connected with the support block in a rotatable way. The middle sections of the support feet are horizontal extension parts and the lower sections of the support feet are support parts, wherein the support block is provided with a support face and magnets are embedded on the support face. The upper surfaces and lower surfaces of the assembled blocks are provided with assembled faces and magnets are embedded on the assembled faces. The outer surfaces of the assembled blocks are provided with mark layers. The assembled blocks are mutually overlaid and arranged on the support face. The demonstration platform, which is a square box body in structure, includes a bottom plate, a left side plate, a right side plate, a back plate, a front plate and a top plate. The demonstration instrument is capable of explaining three main influence factors on the structural stability vividly and conveniently, wherein different assembly methods of the assembled blocks change height of a gravity center and a shape after the assembly and the support feet on the support block are capable of changing the size of the support face. The teaching tool is capable of motivating interest and thinking of students.