A method for searching for a resonant frequency of a transducer, comprising operations of: S1. obtaining a minimum operating frequency and a maximum operating frequency allowed for the transducer; S2. presetting a driving frequency, so as to obtain a hopping-up interval formed between the maximum operating frequency and the preset driving frequency, and a hopping-down interval formed between the preset driving frequency and the minimum operating frequency; S3. obtaining a current phase and a voltage phase of the transducer operating at the preset driving frequency; and when a difference in phase therebetween does not lie within a pre-defined range, if a current leads a voltage in phase, then hopping to the hopping-up interval, or if the current lags the voltage in phase, then hopping to the hopping-down interval, and proceeding with an operation S4; or when the difference in phase between the current and the voltage lies in the pre-defined range, proceeding with an operation S6; S4. setting a new preset driving frequency in a current hopping-up interval or a current hopping-down interval, and so as to obtain a new hopping-up interval and a new hopping-down interval formed between two endpoints of the current hopping-up interval or the current hopping-down interval and a corresponding new preset driving frequency; S5. operating the transducer at the new preset driving frequency; obtaining a current phase and a voltage phase of the transducer operating at the new preset driving frequency in the current hopping-up interval or the current hopping-down interval; and when the difference in phase between the current and the voltage does not lie in the pre-defined range, if the current leads the voltage in phase, then hopping to the hopping-up interval, or if the current lags the voltage in phase, then hopping to the hopping-down interval, and going back to the operation S4; when the difference in phase between the current and the voltage lies in the pre-defined range, proceeding with the operation S6; and S6. obtaining a frequency of the transducer with the difference in phase between the current and the voltage lying in the pre-defined range as the resonant frequency. S1. obtaining a minimum operating frequency and a maximum operating frequency allowed for the transducer; S2. presetting a driving frequency, so as to obtain a hopping-up interval formed between the maximum operating frequency and the preset driving frequency, and a hopping-down interval formed between the preset driving frequency and the minimum operating frequency; S3. obtaining a current phase and a voltage phase of the transducer operating at the preset driving frequency; and when a difference in phase therebetween does not lie within a pre-defined range, if a current leads a voltage in phase, then hopping to the hopping-up interval, or if the current lags the voltage in phase, then hopping to the hopping-down interval, and proceeding with an operation S4; or when the difference in phase between the current and the voltage lies in the pre-defined range, proceeding with an operation S6; S4. setting a new preset driving frequency in a current hopping-up interval or a current hopping-down interval, and so as to obtain a new hopping-up interval and a new hopping-down interval formed between two endpoints of the current hopping-up interval or the current hopping-down interval and a corresponding new preset driving frequency; S5. operating the transducer at the new preset driving frequency; obtaining a current phase and a voltage phase of the transducer operating at the new preset driving frequency in the current hopping-up interval or the current hopping-down interval; and when the difference in phase between the current and the voltage does not lie in the pre-defined range, if the current leads the voltage in phase, then hopping to the hopping-up interval, or if the current lags the voltage in phase, then hopping to the hopping-down interval, and going back to the operation S4; when the difference in phase between the current and the voltage lies in the pre-defined range, proceeding with the operation S6; and S6. obtaining a frequency of the transducer with the difference in phase between the current and the voltage lying in the pre-defined range as the resonant frequency.