In this paper, we propose a novel scheme to form a controllable doublewell opt i cal trap for cold atoms or molecules by using a system composed of a binary π phase plate and a lens illuminated by a plane light wave. We calculate the int ensi ty distribution of the doublewell trap, and study the evolution process of th e optical trap from two wells to a single well, and derive the analytical relation between the characteristic parameters of the doublewell trap (includi ng g eometric parameters, the intensity distributions, intensity gradients and their curvatures) and the parameters of optical system. We find that our doublewell trap can be continuously changed into a singlewell trap by moving the πphas e pl ate along the x direction, and the relation between the distance of the trap ping centers of the two wells and the moving distance of the πphase plate is obt ain ed by data fitting. In addition, our doublewell scheme is not only simple and c onvenient, but also has some important and wide applications in the fields of at omic physics, atom optics, molecule optics and quantum optics and so on.