Current part-scale lattice design methods cause accuracy loss and manufacturability uncertainty in AM preparation stages. STL model conversion and slicing can lead to loss of shape accuracy and surface quality, while unqualified toolpaths may cause printing failures, e.g. pores or re-melting in the powder-bed fusion process. Moreover, all these steps are time-consuming due to the large model file. To solve these challenges, this paper proposes a novel knowledge-based toolpath constructive design method to generate high-precision graded lattice unit cells with manufacturability. It integrates implicit modeling, variable distance field, direct slicing and fine toolpath configuration to construct qualified toolpaths without any intermediate steps. To save computation time in partscale lattice design, predefined different types or sizes of graded lattice unit cells are populated and assembled into a given design space directly. Hence, it has big potential to improve industrial application of part-scale porous structures with fine and gradient porous features.