A VOIP/PSTN telephone line switching device can automatically implement switch on the telephone lines of all rooms of the user between common telephone service and VOIP telephone service in real time. The switching device consists of a DAA module, an SLIC module, a voltage detection circuit, a system logic controller (CPLD), a system logic controller (CPLD), and a power conversion circuit. The user does not need to change the layout of the telephone lines of the user rooms for achieving real-time automatic switch (ordinary telephone high priority) of all rooms between the common telephone and the VOIP telephone, and can correctly detects whether the telephone lines have common telephone service in real time no matter in the process of a call by picking up the telephone or the standby process of the telephone.