The invention discloses a firewall hidden NAT (Network Address Translation) fault judgment method and device. A firewall hidden NAT fault can be judged. The method comprises the steps of S10, carryingout periodic statistics on the number of TCP sessions and the number of UDP sessions of a firewall device, and calculating a ratio of the statistic number of the TCP sessions and the statistic numberof the UDP sessions in a current period; and S11, determining whether a hidden NAT fault exists in the firewall device or not according to the ratio.