A thin optical mechanical keyboard switch comprises a printed circuit board (PCB) (1), a base (2), and a keycap (3). A first accommodation recess (21), a second accommodation recess (22), and a surrounding recess (23) are provided at the base (2). A balancing frame (4) is disposed in the surrounding recess (23). A torsion spring (5) is arranged in the first accommodation recess (21). An upper through hole is provided at a bottom portion of the second accommodation recess (22), and a blocking piece (6), an attachment piece (7), a tension spring (8), and a guiding piece (9) are disposed in the second accommodation recess (22). One end of the tension spring (8) is fastened in the guiding piece (9) and connected to the attachment piece (7), and the other end of the tension spring (8) is connected to the balancing frame (4). The guiding piece (9) comprises an insertion part (91) at a lower end thereof. The insertion part (91) comprises two pieces of insertion plates (911). A lower opening (94) is formed between the two pieces of insertion plates (911), and used to fit the tension spring (8). Communication holes (912) are provided at the two pieces of insertion plates (911), respectively. A lower through hole (11) corresponding to the upper through hole is provided at the PCB (1). A light-emitting transistor (12) and a light-receiving transistor (13) are correspondingly provided at the edge of the lower through hole (11) on the PCB (1). The guiding core (9) slides vertically along a direction of the upper through hole and the lower through hole (11). The thin optical mechanical keyboard switch utilizes light to implement turning on and off functions, realizing a thinned design, and enhancing pressing balance performance and stability.