Phenobenzyme tablets are a blocker of α1 and α2 receptors. The excitation of α receptors leads to smooth muscle relaxation and vasodilatation afterwards, and using this, phenazopyridine is usually needed before pheochromocytoma surgery, which has the effect of dilation. The so-called volume expansion effect of phenazopyridine is that after using phenazopyridine, the peripheral blood vessels of the body expand and the total blood volume of the body increases, so as to avoid the sudden relaxation of the blood vessels of the whole body after the removal of pheochromocytoma and the accumulation of large doses of blood in the peripheral blood vessels of the body, which may lead to hypotensive shock and the situation of failure to rescue. If phenazopyridine is used beforehand to keep a large amount of blood or body fluid in the peripheral blood vessels, it will not lead to sudden hypotension after pheochromocytoma surgery, which takes about two weeks with phenazopyridine.