An analog beamformer used for array antenna and an operating method thereof are provided. The analog beamformer used for array antenna includes an intermediate-frequency amplifying circuit, multiple local oscillators, multiple mixers, multiple radio-frequency amplifying circuits, and a frequency locking circuit. The analog beamformer uses a master-oscillator and multiple slave-oscillators which embed a resonant network of frequency-and-phase-locking. The intermediate-frequency amplifying circuit receives a baseband signal to provide an intermediate-frequency signal. Power supplies or grounding ports of different local oscillators are connected together to provide multiple local-oscillating signals with consistent frequency but different phases. The mixers individually receive the intermediate-frequency signal and one of the local-oscillating signals to provide multiple mixed signals. The radio-frequency amplifying circuits receive the mixed signals to provide multiple radio-frequency signals with consistent frequency but different phases to each antenna. The frequency locking circuit only locks a frequency of one of the local-oscillating signals.