I think the accuracy of measuring the density by Archimedes' principle should not depend on the shape of the material

thaks for your reply, sir.

that means will it give the exact density of my porous sample?

Density measurement indeed doesn't depend on the shape of the sample. The risk in your case is that the liquid you use can penetrate more or less the porosity of your sample, thereby leading to very different results whether air remained trapped in the porosity or not. In your case, I would therefore measure the envelope density with a powder pycnometer, and the true (skeletal) density with a helium pycnometer. With these 2 results, you'll get the exact value of the total porosity (assuming that no closed porosity exists ...)

Alain

Assuming that you do not have "closed porosity" you can use ISO5017:2013 or AS1774.5:2014 where by you weigh your sample dry  and then use a vacuum chamber to evacuate your sample and then introduce water such that it is completely saturated when measured wet...