Abstract: Among free electron laser (FEL) schemes, the Harmonic Lasing Self-Seeded (HLSS) scheme can reduce the radiation bandwidth of self amplified spontaneous emission (SASE) and improve the spectral brightness of FEL in X-ray band. The principle of HLSS has been verified at FLASH, PAL, European XFEL and other laboratories. The HLSS scheme can improve the coherence of a SASE FEL without additional hardware, so it can be easily applied to domestic facilities under construction or running in SASE scheme. In this paper, the principle of the HLSS scheme to achieve narrow bandwidth is summarized, and the quantitative conditions of the undulator parameters are given. Typical parameters of Shenzhen superconducting soft X-ray free electron laser are used to simulate the HLSS scheme. Simulation results show that the bandwidth in the HLSS scheme can be reduced to about 1/2 of the bandwidth in SASE, and the spectral brightness can increase to about 2 times, when the output wavelengths are 4.5 nm and 6.75 nm.