Kant believes that there are only three ways to prove the existence of God through pure reason,namely natural theological proof,cosmological proof and ontological proof,and these three ways can be traced back to the latter in the end. Kant pointed out that pure reason cannot prove the existence of God through the crit-icism of these three proof methods. And Moses Mendelssohn,as the successor of Leibniz-Wolff system,needs to respond to Kant in order to maintain the supremacy of reason. Mendelssohn defends ontological proofs mainly through the distinction be-tween finite and infinite beings and his predicate theory. It is difficult to conclude that Mendelssohn was entirely successful,but he at least weakened the power of Kant's critique.