The invention discloses a measuring device (4) used for measuring the length of cloth in the sewing process. The measuring device is installed on a table board (2) of a sewing machine (1), the measuring device (4) comprises a cover plate (6) and a shell (12), a detecting opening (5) is formed in the cover plate (6), a circuit board system (10), a light source (11), a lens system (8) and an optical sensor (9) are installed in the shell (12), the circuit board system (10) is connected with the light source (11) and the optical sensor (9), and the lens system (8) is provided with a set of lenses emitting light to the detecting opening (5) and another set of lenses reflecting reflected light into the optical sensor (9). The measuring device is simple in structure, high in precision, convenient to install and adjust, and capable of greatly increasing the production efficiency and the processing quality when the measuring device is arranged on various sewing devices in a matched mode.