The invention discloses an open-phase protection circuit composed of voltage comparators, and relates to an open-phase protection circuit which includes a power supply circuit, a sampling circuit, a voltage comparator circuit and a control output circuit, wherein the power supply circuit includes a rectifier transformer, a single phase bridge type rectifier circuit, a filter capacitor a, a stabilizing integrated block and a filter capacitor b; the sampling circuit includes a phase line detecting element a, a phase line detecting element b, a phase line detecting element c, a rectifier diode a, a rectifier diode b, a rectifier diode c, a rectifier diode d, a rectifier diode e, a rectifier diode f, a voltage dividing resistor a and a voltage dividing resistor b; the voltage comparator circuit includes a voltage comparator a, a bias resistor a, a bypass resistor a, a voltage comparator b, a bias resistor b and a bypass resistor b; the control output circuit includes a driving resistor, a triode, a clamping diode, a relay, an interlocking control end a and an interlocking control end b. The open-phase protection circuit provides open-phase protection and low-voltage protection effectively for a three-phase motor, and is simple in structure.