The invention relates to a test circuit for a mechanical switch for a direct current (DC) circuit breaker. The test circuit comprises positive and negative connection ends for connecting a mechanicalswitch sample, and a DC power supply module, an oscillating circuit and a surge voltage generating circuit which are connected in parallel, wherein the DC power supply module, the oscillating circuitand two parallel ends of the surge voltage generating circuit are correspondingly connected to the positive and negative connection ends. The test circuit provides a stable DC current to the mechanical switch, and provides the mechanical switch with the rated current stress and port thermal stress before breaking; provides the mechanical switch with the high-frequency current stress at the breaking moment to enable quenching of arc when zero-crossing during the mechanical switch breaking process; and generates a transient impulse voltage satisfying certain waveform parameters, which is equivalent to the tolerant transient voltage stress between the fractures of the mechanical switch during the breaking process, thereby realizing the dynamic insulation test of the mechanical switch under rated operating conditions, and the equivalence of the rated current stress before the breaking, the high-frequency current stress during the breaking, and the transient voltage stress after the breaking between the ports. The test circuit is simple in method and strong in stress equivalence.