This paper presents a new 1D Neutronics/Thermal-hydraulics code ATAC-1D based on the advanced Jacobian-Free Newton-Krylov (JFNK) method and the low dimensional equivalent strategy. Conventional operator-splitting (OS) strategies are used to maintain its accuracy with small time steps and linearization of the nonlinear problem, which leads to slow computation speed and linearization error. The JFNK method solves the troubles in the coupled neutronics/thermal-hydraulics problems mentioned above. Furthermore, a core-wide three dimension to one dimension equivalent method has been developed to provide variable few-group parameters. Finally, the performance of the coupled neutronics/thermal-hydraulics code ATAC-1D is studied by simulating four OECD/NEA CRP PWR rod ejection benchmark problems. The simulation results are compared to the reference ones, which proves that the developed 1D code has a good accuracy and practicability in nuclear reactor transient calculation. (C) 2016 Elsevier Ltd. All rights reserved.