In the paper, constrained approximate controllability for linear dynamical systems described by abstract differential equations with unbounded control operator is considered. Using methods of spectral analysis for linear self-adjoint operators and general constrained controllability results given in the paper [20], necessary and sufficient conditions of the constrained approximate controllability for the piecewise polynomial controls with values iii a given cone are formulated and proved. Moreover, as illustrative examples constrained approximate boundary controllability of one-dimensional distributed parameter dynamical systems described by partial differential equations of parabolic type with Dirichlet and Neumann boundary conditions are investigated. The constrained controllability conditions obtained in the paper represent an extension of the unconstrained controllability results given in [5] and [6].