The invention relates to an interlocked type medium-frequency power source main unit and auxiliary unit automatic switching circuit. According to the interlocked type medium-frequency power source main unit and auxiliary unit automatic switching circuit, one set of multi-contact relays is adopted to switch on main unit and auxiliary unit starting circuits in a medium-frequency power source and is made to maintain a self-hold state; the other set of multi-contact relays is adopted to control the output enable circuits of the main unit and auxiliary unit of the medium-frequency power source; and the control contact of the auxiliary unit enable circuit and the state contact of the main unit are connected in series into the main unit enable circuit, and the control contact of the main unit enable circuit and the state contact of the auxiliary unit are connected in series to the auxiliary unit enable circuit, and therefore, the enable functions of the main unit and the auxiliary unit can form an interlocked type circuit structure, and thus, the problem of main unit and auxiliary unit paralleled simultaneous output can be avoided, and the enable of a fault unit can be automatically switched off under the fault state of the medium-frequency power source, and the power supply security of the system can be ensured.