In this study, a plug flow reactor was built to investigate iron sulfide scale precipitation at various temperatures, pH and ionic strength conditions and two pieces of carbon steel C1018 coupons were put inside as reaction surfaces. The ferrous ion and total sulfide in collected effluent samples were measured to determine precipitation kinetics and solubility. The solid that formed on the steel surfaces were analyzed by Scanning Electron Microscopy (SEM/EDS) and X-ray Diffraction (XRD). The solubility data from this study and literature were collected and fitted by Matlab to build up a reliable FeS solubility prediction model. The experimental results show that mackinawite is the predominant precipitated scale and could be stable for a week at pH higher than 6.0. Iron sulfide precipitation is under diffusion control, accelerated by high temperature and ionic strength. At pH 6 – 7, the aqueous phase neutral species, such as 𝐹𝑒𝑆0𝑎𝑞, plays an important role in the solubility and precipitation kinetic. Based on this study, a new solubility model that combines Pitzer theory and ion-complexes (speciation of ferrous ion) has been developed for iron sulfide solubility calculation and scale prediction.