A quick reciprocating wrench, comprising: a main body comprising a concavely-shaped first jaw opening, the first jaw opening comprising a pivot end on one side and a block portion on another side in opposite to the pivot end, the block portion comprising a block face; a claw comprising a concavely-shaped second jaw opening, the second jaw opening comprising a pivotal combination portion on one side and a retaining portion on another side in opposite to the pivotal combination portion, the retaining portion comprising an outer retaining face and an opposite inner retaining face, the pivotal combination portion of the claw being pivotally connected with the pivot end of the main body along a pivot center, such that the claw and the main body pivotally move with respect to each other; and an elastic device disposed between the main body and the claw; when the wrench is not engaged with a fastener, the first jaw opening of the main body and the second jaw opening of the claw form a sealing status; when the wrench is engaged with the fastener, the fastener is placed in the first jaw opening and the second jaw opening; a distance between the block portion and a neighboring first force bearing face of the fastener decreases in a direction in opposite to an expansion direction of the claw with respect to the block portion; a largest width of a distal end of the retaining portion is greater than a smallest distance between the block portion and the neighboring first force bearing face of the fastener; a first inclination angle (θ1) is included by the block face and the first force bearing face, and a second inclination angle (θ2) is included by the outer retaining face and the first force bearing face, and the second inclination angle (θ2) is smaller than the first inclination angle (θ1); when the wrench is engaged with the fastener during a fastening operation, a counterforce of the first force bearing face of the fastener forces the retaining portion to expand to resist against the block portion; the retaining portion resiliently expands, such that the longest distance between the outer retaining face and the pivot center is greater than the shortest distance between the block portion and the pivot center, so that the block face and the first force bearing face form a block effect with the retaining portion, so as to prevent the retaining portion from detaching with respect to the block portion. a main body comprising a concavely-shaped first jaw opening, the first jaw opening comprising a pivot end on one side and a block portion on another side in opposite to the pivot end, the block portion comprising a block face; a claw comprising a concavely-shaped second jaw opening, the second jaw opening comprising a pivotal combination portion on one side and a retaining portion on another side in opposite to the pivotal combination portion, the retaining portion comprising an outer retaining face and an opposite inner retaining face, the pivotal combination portion of the claw being pivotally connected with the pivot end of the main body along a pivot center, such that the claw and the main body pivotally move with respect to each other; and an elastic device disposed between the main body and the claw; when the wrench is not engaged with a fastener, the first jaw opening of the main body and the second jaw opening of the claw form a sealing status; when the wrench is engaged with the fastener, the fastener is placed in the first jaw opening and the second jaw opening; a distance between the block portion and a neighboring first force bearing face of the fastener decreases in a direction in opposite to an expansion direction of the claw with respect to the block portion; a largest width of a distal end of the retaining portion is greater than a smallest distance between the block portion and the neighboring first force bearing face of the fastener; a first inclination angle (θ1) is included by the block face and the first force bearing face, and a second inclination angle (θ2) is included by the outer retaining face and the first force bearing face, and the second inclination angle (θ2) is smaller than the first inclination angle (θ1); when the wrench is engaged with the fastener during a fastening operation, a counterforce of the first force bearing face of the fastener forces the retaining portion to expand to resist against the block portion; the retaining portion resiliently expands, such that the longest distance between the outer retaining face and the pivot center is greater than the shortest distance between the block portion and the pivot center, so that the block face and the first force bearing face form a block effect with the retaining portion, so as to prevent the retaining portion from detaching with respect to the block portion.