A method and associated apparatus using delay correlation for determining whether an input signal is a target signal is provided. The method includes sampling the input signal to generate 2N sample values, the sample values having a period N, where N is a positive integer; calculating the 2N sample values to obtain a first value according to a first operation method; calculating the 2N sample values to obtain a second value according to a second operation method; obtaining a determination value according to the first value and the second value; and determining whether the input signal is the target signal according to the determination value and a threshold.