A method for startup of a crystal oscillator (XO) with aid of external clock injection, associated XO and a monitoring circuit therein are provided. The XO includes an XO core circuit, an external oscillator, and an injection switch, where a quality factor of the external oscillator is lower than a quality factor of the XO core circuit. The method includes: utilizing the external oscillator to generate an injected signal; turning on the injection switch to make energy of the injected signal be injected into the XO core circuit, where an amplitude modulation (AM) signal is generated according to combination of the injected signal and an intrinsic oscillation signal from the XO core circuit; and controlling the external oscillator to selectively change an injection frequency of the injected signal according to the AM signal. More particularly, the injection switch is not turned off until the startup process is completed.