Conventional wastewater analysis based on on-site sampling holds a limitation of accuracy because of fluctuation of influent composition in time and its corruptible nature in organics. To cope with the problem, this study aimed at developing an alternative method using on-site lab-scale activated sludge reactors, where a set of mean influent material concentrations was calculated from the analysis of activated sludge constituents of which fluctuation was damped due to long sludge retention time. Focusing on the activated sludge collected from the reactor having a primary settling tank, the soluble biodegradable material concentrations in the influent were calculated using IWA Activated Sludge Model. Similarly the concentrations of inert and biodegradable particulates in the influent were obtained from the increment of activated sludge constituents between the reactor without a primary settling tank and that with a primary settling tank. The specific decay rate of the activated sludge, which was an influential kinetic parameter on the mathematical calculation, was also regularly monitored. According to the statistical analysis, 6 collection frequencies seemed to be enough to grab the parameter. This new method required noticeably low manpower to estimate particulate BOD, soluble BOD, SS and VSS in the wastewater compared to the conventional techniques.