The present invention discloses an acoustic sensor for rock crack detection including an acoustic emission probe, a probe installation mechanism and a transmission mechanism which transmits a combination of the probe installation mechanism and the acoustic emission probe to a setting position inside a borehole in the monitored rock mass. The acoustic emission probe installation mechanism essentially includes a sleeve component, a guide component, an end cap and springs. The guide component is in pluggable connection with the sleeve component or the end cap. The assembled probe installation mechanism locks the acoustic emission probe in a probe sleeve of the sleeve component. The springs inside the elastic sleeve of the sleeve component are in a compressed state.