A novel dynamic optical coupler is presented by employing a specially designed Dammann grating. By controlling the shift parameter of Dammann grating, this device can easily realize the function of either a beam splitter or a combiner and transformation between both of them. An arbitrary N×M dynamic optical coupling can be achieved by choosing an appropriate Dammann grating. In the experiment a high performance 1×8 dynamic coupler achieves good uniformity of 0.03, insertion loss of around 10.5 dB for each channel as a splitter and a low insertion loss of 0.43 dB as a switch at the wavelength of 1550 nm. This device has the advantages of easy adjustment, compact size and low loss. Moreover, Dammann grating used in the experiment can be easily mass-fabricated by binary optical technology. In addition, this system is a preferable approach in the integral packaging. This device has the unique diffractive advantages of Dammann-grating-incorporated system for dynamic optical coupling of a large fiber array. Note that this method is generally applicable and should be interesting for practical use.