The present disclosure relates to a shoulder adjustment mechanism for a seat backrest, comprising: an upper backrest frame () and a lower backrest frame () in an up-and-down arrangement, each of the upper backrest frame () and the lower backrest frame () including a first side and a second side opposing to each other in a left-right direction, a third side facing a seat, and a fourth side facing away from the seat; a pair of first hinges () hingedly connected to the first sides and the second sides of the upper backrest frame () and the lower backrest frame () and close to the third sides; a pair of second hinges () hingedly connected to the first sides and the second sides of the upper backrest frame () and the lower backrest frame () and close to the fourth sides, the lower backrest frame () being provided with sliding grooves (); a first base () fixed to the lower backrest frame (); a first transmission structure () rotatably mounted on the first base () and in driving connection with a motor, wherein a spacing exists between a rotating position of the first transmission structure () and a fixed position of the first base (); a first lead screw () in transmission connection with the first transmission structure () and connected with the second hinge (), wherein the motor drives the first lead screw () to move through the first transmission structure (), thereby bringing the second hinges () to move in the sliding grooves ().
