A method for determining an ultrasonic detecting cycle is provided. Firstly, an initial detecting cycle T is set. Then, a first sensing wave is generated. Then, a first time-of-flight value is calculated corresponding to the first sensing wave. After the first sensing wave has been generated for the initial detecting cycle T, a second sensing wave is generated. Then, a second time-of-flight value is calculated corresponding to the second sensing wave. Afterwards, the second time-of-flight value is compared with the first time-of-flight value. If a difference between the second time-of-flight value and the first time-of-flight value is smaller than a threshold value, the initial detecting cycle T is determined as the ultrasonic detecting cycle.