The effect of cost function on phase-locked based on dithering technique is studied. The cost function is often obtained by collecting the energy in a pinhole, and the diameter of the pinhole has a strong effect on the sign and value of the cost function. The effects of the fill factor of laser array, the number of laser beam, the radius of the light spot and wavelength on the best cost function are analysed in detail. It is found that the fill factor of laser array and the number of laser beams have a little effect on the best diameter of the pinhole for positive cost function which is located on 0.7~0.8 of the width of the main lobe of far field pattern. The radius of the light spot and wavelength have no effect on the best diameter of the pinhole. The best diameter of the pinhole for negative cost function will increase along with the number of laser beams increasing, but the fill factor of laser array has no effect on it.