Conservation is a basic law in the field of physics. The momentum conservation and the mass energy conservation are two important laws. Mass is solidified energy and energy is invisible mass. They are two different forms of matter. Momentum, like energy, is an objective existence. It is the manifestation of material existence and will not disappear out of thin air. Lorentz transformation is only a coordinate system transformation, which belongs to the observer effect and cannot be used to calculate the changes of inherent physical quantities of matter, such as the mass and energy of matter. Combining momentum with energy, the energy-momentum is defined, which is a new perspective of conservation. This leads to the redefinition of the dynamic mass and the dynamic energy. From this, the law of conservation of the energy-momentum, the law of conservation of the dynamic energy, the law of conservation of the dynamic mass and the formula of energy, mass and momentum can be derived. The analysis shows that the dilemma of interpretations of several representative experiments can be satisfactorily explained within the framework of the energy-momentum.