Natural pumice was used as coarse aggregate, and lightweight aggregate concrete with strength grade of LC40 was used for reference to investigate the effects of limestone powder added on the compression strength, flexural strength and axial compressive strength of natural lightweight aggregate concrete. Stress strain curve constitutive equation was established, and effects of limestone powder content on internal structures of specimens were studied using scanning electron microscope(SEM). The results demonstrate that the addition of limestone powder signifacantly improves basic mechanical properties of natural lightweight aggregate concrete. It is concluded that the mechanical property of lightweight aggregate concrete reaches the best with addition of 30%(by mass) limestone powder. The constitutive equation for lightweight aggregate concrete is accurate if expressed with rational equation.