A mechanical analytical model is put forward for the three-dimensional isotropic reticulated porous metals under biaxial tension according to a simplified structural model in this study. The relationship between biaxial nominal stresses and porosity at the beginning of breakdown is derived for these materials from this analytical model. Based on the resultant relationship, the safety criterion can be further obtained for these foamed metals under biaxial loading. When the biaxial nominal stresses are equal to each other, the mathematical relationship between loading strength and porosity can be also gained for the foamed metals under biaxial equal loads. According to the typical experiment on nickel foam as an example, it is proved that these theoretical formulas well accorded with the practical ones. The safety criterion for the porous bodies under biaxial loading can be gotten from this model, which is not dealt with or difficult to do by other relative theories, such as Gibson-Ashby model.