A constant torque module (1, 1a, 1b) for a power tool having an output portion, the constant torque module (1, 1a, 1b) comprising: a transmission ring member (10, 10a, 10b) including a main body, a coupling portion (12, 12a, 12b) extending from the main body for being coupled with the output portion, a core shaft extending from a lateral side of the main body opposite to the coupling portion (12, 12a, 12b), two accommodations provided at the main body and located at two lateral sides of the core shaft, and two drive blocks extending from the main body in a way that each of the two drive blocks is farther from the core shaft than each of the two accommodations is; two buffer units, each of which is disposed in a part of one of the two accommodations such that each of the two accommodations remains a receiving space; an internal shaft member including an inner disc body, two inner protrusions extending from the inner disc body into the receiving spaces respectively, a shaft notch provided at the inner disc body and engaged with the core shaft of the transmission ring member (10, 10a, 10b), a shaft rod extending from a lateral side of the inner disc body opposite to the two inner protrusions, and an engagement portion provided at a terminal end of the shaft rod; and an external shaft member including an outer disc body, two outer protrusions extending from the outer disc body towards the main body of the transmission ring member (10, 10a, 10b) and located outside an outer periphery of the inner disc body, a shaft tube extending from a lateral side of the outer disc body opposite to the two outer protrusions, a drive portion provided at a terminal end of the shaft tube, a shaft hole recessed from the outer disc body into an inside of the shaft tube and inserted with the shaft rod of the internal shaft member, and an engagement notch provided at an inner end of the shaft hole and engaged with the engagement portion of the internal shaft member; wherein the inner disc body is located between the outer disc body and the main body of the transmission ring member (10, 10a, 10b); wherein when the transmission ring member (10, 10a, 10b) is rotated in a first direction, the two drive blocks are in contact with the two outer protrusions respectively to transmit a rotational force to the external shaft member; wherein when the transmission ring member (10, 10a, 10b) is rotated in a second direction reverse to the first direction, the two buffer units are in contact with the two inner protrusions respectively to transmit a reverse rotational force to the internal shaft member and then the external shaft member through the engagement portion and the engagement notch. a transmission ring member (10, 10a, 10b) including a main body, a coupling portion (12, 12a, 12b) extending from the main body for being coupled with the output portion, a core shaft extending from a lateral side of the main body opposite to the coupling portion (12, 12a, 12b), two accommodations provided at the main body and located at two lateral sides of the core shaft, and two drive blocks extending from the main body in a way that each of the two drive blocks is farther from the core shaft than each of the two accommodations is; two buffer units, each of which is disposed in a part of one of the two accommodations such that each of the two accommodations remains a receiving space; an internal shaft member including an inner disc body, two inner protrusions extending from the inner disc body into the receiving spaces respectively, a shaft notch provided at the inner disc body and engaged with the core shaft of the transmission ring member (10, 10a, 10b), a shaft rod extending from a lateral side of the inner disc body opposite to the two inner protrusions, and an engagement portion provided at a terminal end of the shaft rod; and an external shaft member including an outer disc body, two outer protrusions extending from the outer disc body towards the main body of the transmission ring member (10, 10a, 10b) and located outside an outer periphery of the inner disc body, a shaft tube extending from a lateral side of the outer disc body opposite to the two outer protrusions, a drive portion provided at a terminal end of the shaft tube, a shaft hole recessed from the outer disc body into an inside of the shaft tube and inserted with the shaft rod of the internal shaft member, and an engagement notch provided at an inner end of the shaft hole and engaged with the engagement portion of the internal shaft member; wherein the inner disc body is located between the outer disc body and the main body of the transmission ring member (10, 10a, 10b); wherein when the transmission ring member (10, 10a, 10b) is rotated in a first direction, the two drive blocks are in contact with the two outer protrusions respectively to transmit a rotational force to the external shaft member; wherein when the transmission ring member (10, 10a, 10b) is rotated in a second direction reverse to the first direction, the two buffer units are in contact with the two inner protrusions respectively to transmit a reverse rotational force to the internal shaft member and then the external shaft member through the engagement portion and the engagement notch.