This invention discloses a protection circuit in a ballast when no lamp tube is cut-in including a first comparator, its input positive terminal samples the supply lead-in end voltage of the second filament of the lamp, the output end is connected to the supply of the oscillation drive circuit of the lamp by a MOS tube, the lead-out end of the second filament is serial to a risistor then to connect the earth. When the lamp works normally, the input negative voltage of the first is higher than the positive and low level is output , the MOS tube is stopped, the oscillation drive circuit works normally. When there is no lamp in the circuit the input negative voltage of the first is lower than the positive and high level is output, MOS is turned on, its source voltage is zero, the supply voltage of the drive circuit is zero and stops oscillating, the circuit is protected.