本文把惯性运动的定义由单质点的“m(?)=常量”扩大为适用于质点系的“M(?)=常量、(?)_c=常量、T_r=常量。”本文指出了170多年以前保留至今的虚位移原理是不充分的,并根据惯性运动的新定义重证了这一原理。从而扩大了原理的内容和它的适应范围。本文指出了惯性有两种表现,惯性运动既是惯性运动的组合又是非惯性运动的组合. In this paper, the definition of inertial motion is extended from the “m(?)=constant” of a single mass point to “M(?)=constant, (?)_c=constant, T_r=constant.” The principle of virtual displacement, which has been retained until now, is insufficient, and this principle is reconfirmed according to the new definition of inertial motion. Thus expanding the content of the principle and its scope of adaptation. This paper points out that there are two manifestations of inertia, inertial motion is both a combination of inertial motion and a combination of non-inertial motions.