Front-haul is an important sub-system in 5G centralized, collaborative, cloud, and clean radio access network (C-RAN), which connects base stations and distributed units. However, it is still a challenge to achieve manageable cost-effective front-haul supporting 12-channel transmission over 10 km G.652.D fibre. Currently, a novel front-haul approach based on so-called medium wavelength-division multiplexing (MWDM) is in the process of standardization in various international organizations, such as ITU-T and O-RAN. In this article, we provide a tutorial that systematically overviews the semi-active WDM front-haul based on multi-carrier pilot tone operation, administration and maintenance (OAM), and MWDM. Firstly, the semi-active WDM architecture based on multi-carrier pilot-tone is introduced. Afterwards, the principle of 12-channel bidirectional MWDM system is discussed. The MWDM scheme is performed by shifting design parameters of the optical grate of the coarse WDM (CWDM) direct modulation laser (DML) at the blue side and the red side, respectively, which can reuse the low-cost and mature industry chain of 25-Gb/s 6-channel CWDM. Next, the architecture of centralized and software defined network (SDN)-based management and control system for semi-active WDM is also presented. Finally, real-time transmission of 300 Gb/s 12-channel MWDM-based semi-active system over 10-km are achieved in lab test and field trial, demonstrating the feasibility and performance of the MWDM-based semi-active solution. We believe that this work might greatly promote the practical applications of 5G C-RAN front-haul.