Ultra-narrow linewidth and single-mode output are vital for the further development of semiconductor nanolasers capable of excellent lasing performance. This paper addresses a systematic linewidth narrowing method for the design of a new type of nanoplasmonic laser coupled with distributed feedback (DFB) structure of nanogratings. Based on the physical model of DFB nanoplasmonic laser, the optical field distribution, carrier density distribution, relaxation oscillation and lasing spectra of the DFB laser were simulated with different parameters. The optimal device design for the DFB nanoplasmonic laser was achieved through a complete numerical simulation based on the multiple effects. By optimizing the influence of device parameters, the optimal structures for the DFB nanoplasmonic laser were achieved with the optimal optical properties of cavity mode, spectra and output power. The monochromatic DFB nanoplasmonic laser functions the optimum lasing performance with a single-mode peak at 514 nm, 3 dB linewidth of 0.1 pm, and the threshold current of 16 mA. This work provides a fundamental and technical support for the development of a new generation of efficient nanolaser devices with good spectra merits.