The calculation method for profile curves in a kind of cycloidal pump was proposed based on meshing principle. According to the relative motion of the rotors and coordinate transformation method, the profile coordinates of cycloidal gear were obtained. The profile coordinates of cylindrical ring gear were accurately calculated in accordance with the meshing region and the non-meshing region. The example of design and calculation was also given in this paper. According to the results, the profile curve of rotors was drawn.