The open quantum system plays an important role in quantum information, quantum computation, quantum chemistry, condensed matter physics, and biophysics. The most common equations to describe open quantum systems include the quantum master equation(QME), the stochastic Schrodinger equation(SSE), and the quantum Fokker Planck equation(QFPE). However, these equations only simulate high temperatures and weak coupling. In order to simulate low temperatures and strong coupling, the stochastic Liouville-Von Neumann equation(SLNE) and the hierarchical equations of motion(HEOM) have been derived by J.T. Stockburger and Tanimura respectively to solve this problem. In this lecture, we will first discuss the semiclassical limit of the Lindblad equation(Markovian quantum master equation), then we will show how to derive all of the above equations under the unified framework of the path integrals method of the open quantum system. This method can help us see the connections between these different equations and their respective advantages and disadvantages better.