Scale inhibitors have been widely used for scale control in oil and gas production. The accurate measurement of residual scale inhibitor concentration in the produced brine is essential for scale prevention. However, these scale inhibitors are effective at sub-stoichiometric concentration in most production conditions (e.g., 1 mg/L active concentration, or even lower). It is rather difficult to measure such low inhibitor concentration with traditional ICP-OES/MS or IC method, especially for non- or low- phosphorous polymeric scale inhibitors. Furthermore, the combo of scale inhibitors and corrosion inhibitors are used in the most application, which requires the measurement of effective scale inhibitor concentration. Therefore, there is a high demand of a fast, sensitive, universal and cheap method to determine the effective scale inhibitor concentration in complicated field brines. In this study, a new assay method is developed to determine the effective scale inhibitor concentration. This assay method is based upon the linear relationship between the effective inhibitor concentration and the critical time of barite scale formation, which is determined by turbidity measurement using a CSTR apparatus in Brine Chemistry Inhibitor (BCIn) method. This linear relationship has been validated by experimental observation. The recommended procedures for the sample preparations from the real oilfield brine were also developed to help in the quick setup of the measurement. Various types of inhibitors (i.e., SPCA, PPCA, PVS) have been tested with different sample types (i.e., in the synthetic brines, laboratory samples and real oilfield produced brine). The new assay method is quick, robust and accurate with the relative error less than 10% even at 1 mg/L of inhibitor, which indicating a limit of detection (LODs) around 0.1 mg/L for most cases.