Abstract: The development on a parallel 2.5D electromagnetic and particle-in-cell code named NEPTUNE2D is introduced preliminarily. The code is developed on the J parallel adaptive structured mesh applications infrastructure (JASMIN), with high parallel efficiency, strong expansibility and adaptive load-balance. A new PIC method is used instead of the classical one, which is more suitable for massively parallel simulation. This code can construct and simulate devices in the R-Z coordinate, and it is capable of fast simulation and design of high power microwave (HPM) devices and electron-vacuum tubes. At present, basic physical function modules are accomplished, such as electromagnetic field update, particle movement, electromagnetic wave input/output and particle injection/absorption. This code is verified and validated by simulations of a coaxial-line, a cylindrical waveguide, a coaxial diode and a foilless diode. Finally, by using this code, a CRBWO (coaxial relativistic backward wave oscillator) with high efficiency is designed and simulated. The physical figures are presented and parallel efficiency is given.