Abstract: Virtual displacement is an important physical concept in physics.For exaggerating the role of the non-reality in applications of the virtual displacement，we analyze how the virtual displacement is applied，which can be divided into three steps: 1) for a given physical problem，the definition of virtual displacement of a variable is introduced; 2) the variation of a mathematical function along the change of the virtual displacement is given，which is not necessarily compatible with physics principles; 3) with imposing physical conditions on the mathematical function，the physics consequence is resulted.

Key words: analytical mechanics, virtual displacement, thermodynamics