Abstract: National Institute of Metrology (NIM) has realized the ultra-stable laser systems of hundred Hz, Hz and sub Hz magnitudes in 2008, 2011, and 2018. This paper focuses on the methods and conditions for the realization of ultra-stable lasers of various indices, discusses the reference cavity, fast feedback locking technique, and precision control technique to suppress the influence of external environment on the length of the reference cavity, and obtain 10⁻¹⁵ or higher frequency stability of the ultra-stable laser. Experimentally, an external cavity diode laser (ECDL) is locked to the 30-cm-long high-finesse ULE reference cavity, and the frequency stability of 3×10⁻¹⁶ is achieved.