The clock signal detection circuit includes a lock detection circuit, a duty cycle detection circuit, a first logic circuit, and a counter. The lock detection circuit detects whether an input clock signal and a feedback clock signal of a delay locked loop are in phase. The duty cycle detection circuit detects whether the duty cycle of the input clock signal is within a percentage range. The first logic circuit, electrically connected to the lock detection circuit and the duty cycle detection circuit, outputs a detecting result signal which is at first logic level when the input clock signal are in phase with the feedback clock signal, and the duty cycle of the input clock signal is within a percentage range. The counter outputs a lock detection signal which is at the first logic level when the detecting result signal has maintained at the first logic level for a first constant period of time.