An optical measurement system including a two-dimensional sine wave annuls grating and a measurement unit is provided. The two-dimensional sine wave annuls grating includes a rotary shaft and a plurality of sine wave structures surrounding the rotary shaft and continuously arranged. The measurement unit is adapted to output a light beam towards the two-dimensional sine wave annuls grating, wherein each of the sine wave structures is adapted to reflect the light beam from the two-dimensional sine wave annuls grating back to the measurement unit. A measurement method for errors of a rotating platform and a two-dimensional sine wave annuls grating are also provided.