"动量不变原理" can be translated into English as "the principle of conservation of momentum" or "the invariance of momentum principle".
The former, "the principle of conservation of momentum," is more commonly used in physics to describe the idea that the total momentum of a closed system remains constant unless acted upon by an external force. The latter, "the invariance of momentum principle," emphasizes the unchanging nature of momentum in a given context, though it is less frequently encountered in standard physics terminology.
In most cases, "the principle of conservation of momentum" is the preferred and most accurate translation.